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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2019-08-06T23:43:14.0059713+00:00 -->
<!-- Version            : 3.6.7150.26138 -->
<!-- Package ID         : 0c4e3a62d0594bb5a5c33db4874a0769 -->
<!-- Copyright (c) 2019 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="slno_AssetAcquisitionContingentConsiderationLiabilityNoncurrent" name="AssetAcquisitionContingentConsiderationLiabilityNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_DocumentAndEntityInformationAbstract" name="DocumentAndEntityInformationAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_MinorityInterestInvestmentInSubsidiary" name="MinorityInterestInvestmentInSubsidiary" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_SeriesAWarrantsMember" name="SeriesAWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_TwoThousandSeventeenPIPEWarrantLiabilityMember" name="TwoThousandSeventeenPIPEWarrantLiabilityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_TwoThousandEighteenPIPEWarrantLiabilityMember" name="TwoThousandEighteenPIPEWarrantLiabilityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_AssetAcquisitionContingentConsiderationChangeInFairValue" name="AssetAcquisitionContingentConsiderationChangeInFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_IncomeLossFromMinorityInterestInvestments" name="IncomeLossFromMinorityInterestInvestments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AdjustmentsToAdditionalPaidInCapitalStockIssuedIncreaseDecreaseIssuanceCosts" name="AdjustmentsToAdditionalPaidInCapitalStockIssuedIncreaseDecreaseIssuanceCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AssetPurchaseAxis" name="AssetPurchaseAxis" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_AssetPurchaseDomain" name="AssetPurchaseDomain" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_EssentialisIncMember" name="EssentialisIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_CashAndCashEquivalentsPeriodIncreaseDecreaseIncludedInCurrentAssetsHeldForSale" name="CashAndCashEquivalentsPeriodIncreaseDecreaseIncludedInCurrentAssetsHeldForSale" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_WarrantLiabilityDisclosureTextBlock" name="WarrantLiabilityDisclosureTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_AssetAcquisitionsDisclosureTextBlock" name="AssetAcquisitionsDisclosureTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_SeriesCWarrantsMember" name="SeriesCWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_WorkingCapital" name="WorkingCapital" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_LiquidityAndManagementsPlansLineItems" name="LiquidityAndManagementsPlansLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_LiquidityAndManagementsPlansTable" name="LiquidityAndManagementsPlansTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_ProceedsFromIssuanceOrSaleOfEquityGross" name="ProceedsFromIssuanceOrSaleOfEquityGross" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_CommonStockWarrantLiabilityMember" name="CommonStockWarrantLiabilityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_PurchasePriceContingentLiabilityMember" name="PurchasePriceContingentLiabilityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_ClassOfWarrantOrRightWarrantTerm" name="ClassOfWarrantOrRightWarrantTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_SeriesAWarrantsToPurchaseCommonStockMember" name="SeriesAWarrantsToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_SeriesBWarrantsMember" name="SeriesBWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_TwoThousandSeventeenPIPEWarrantLiabilityToPurchaseCommonStockMember" name="TwoThousandSeventeenPIPEWarrantLiabilityToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_TwoThousandEighteenPIPEWarrantLiabilityToPurchaseCommonStockMember" name="TwoThousandEighteenPIPEWarrantLiabilityToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_ClassofWarrantOrRightLiquidatedDamagesAmountPercentofVolumeWeightedAveragePrice" name="ClassofWarrantOrRightLiquidatedDamagesAmountPercentofVolumeWeightedAveragePrice" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_WarrantsExercised" name="WarrantsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_DerecognitionOfWarrantDerivativeLiability" name="DerecognitionOfWarrantDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_TenderOfferExpirationDate" name="TenderOfferExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_StockIssuedDuringPeriodSharesCashlessExerciseOfWarrants" name="StockIssuedDuringPeriodSharesCashlessExerciseOfWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_CashlessWarrantsExercised" name="CashlessWarrantsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_ClassOfWarrantOrRightExercisePriceDescription" name="ClassOfWarrantOrRightExercisePriceDescription" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_LesseeLeasingArrangementsOperatingLeasesIncreasestoMonthlyPaymentsTerm" name="LesseeLeasingArrangementsOperatingLeasesIncreasestoMonthlyPaymentsTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_AdditionalOperatingLeaseOfficeSpace" name="AdditionalOperatingLeaseOfficeSpace" type="num:areaItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_InvestmentThresholdRequirement" name="InvestmentThresholdRequirement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_InvestmentThresholdRequirementStockIssuance" name="InvestmentThresholdRequirementStockIssuance" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_OAHLMember" name="OAHLMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_CapniaIncJointVentureMember" name="CapniaIncJointVentureMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_CosenseNFIandSerenzMember" name="CosenseNFIandSerenzMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_PercentageOfDiscountForLackOfControl" name="PercentageOfDiscountForLackOfControl" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_IncreaseDecreaseInMinorityInterestInvestmentInSubsidiary" name="IncreaseDecreaseInMinorityInterestInvestmentInSubsidiary" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_DisposalGroupIncludingDiscontinuedOperationProductRevenue" name="DisposalGroupIncludingDiscontinuedOperationProductRevenue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_DisposalGroupIncludingDiscontinuedOperationCostOfGoodsSoldRefunded" name="DisposalGroupIncludingDiscontinuedOperationCostOfGoodsSoldRefunded" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_DisposalGroupIncludingDiscontinuedOperationResearchAndDevelopment" name="DisposalGroupIncludingDiscontinuedOperationResearchAndDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_DisposalGroupIncludingDiscontinuedOperationSalesandMarketing" name="DisposalGroupIncludingDiscontinuedOperationSalesandMarketing" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_DisposalGroupIncludingDiscontinuedOperationCostAndExpense" name="DisposalGroupIncludingDiscontinuedOperationCostAndExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="slno_OwnershipInterestPercentageofVotingRightsofStock" name="OwnershipInterestPercentageofVotingRightsofStock" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="slno_IncentiveStockOptionsMember" name="IncentiveStockOptionsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_SeriesDWarrantsMember" name="SeriesDWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_WarrantsToPurchaseCommonStockMember" name="WarrantsToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_UnderwriterMember" name="UnderwriterMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_TwoThousandAndTenAndTwoThousandAndTwelveConvertiblePromissoryNotesMember" name="TwoThousandAndTenAndTwoThousandAndTwelveConvertiblePromissoryNotesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantGranted" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantGranted" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantExercisedInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantExercisedInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantForfeituresInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantForfeituresInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesAvailableForGrantRollForward" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesAvailableForGrantRollForward" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" name="SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedinPeriodWeightedAverageRemainingContractualTerm" name="SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedinPeriodWeightedAverageRemainingContractualTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_ClassOfWarrantOrRightWarrantsExpirationDate" name="ClassOfWarrantOrRightWarrantsExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_ClassOfWarrantOrRightWarrantsExpirationMonthAndYear" name="ClassOfWarrantOrRightWarrantsExpirationMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_SeriesCWarrantToPurchaseCommonStockMember" name="SeriesCWarrantToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_SeriesDWarrantToPurchaseCommonStockMember" name="SeriesDWarrantToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
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  <xsd:element id="slno_AssetAcquisitionsAbstract" name="AssetAcquisitionsAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_AssetAcquisitionLineItems" name="AssetAcquisitionLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_AcquisitionsTable" name="AcquisitionsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_EarnoutPaymentsMember" name="EarnoutPaymentsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="slno_AssetAcquisitionFairValueOfStockConsideration" name="AssetAcquisitionFairValueOfStockConsideration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AssetAcquisitionFairValueOfCashConsideration" name="AssetAcquisitionFairValueOfCashConsideration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AssetAcquisitionConsiderationTransferred1" name="AssetAcquisitionConsiderationTransferred1" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AssetAcquisitionAssumedDeferredTaxLiabilities" name="AssetAcquisitionAssumedDeferredTaxLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AssetAcquisitionContingentConsiderationLiabilityMeasurementInput" name="AssetAcquisitionContingentConsiderationLiabilityMeasurementInput" type="xbrli:decimalItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
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  <xsd:element id="slno_SecondCommercialSalesMilestone" name="SecondCommercialSalesMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_ContingentLiabilityPaymentOnAchievementOfSalesMilestone" name="ContingentLiabilityPaymentOnAchievementOfSalesMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="slno_AreaOfOperatingLease" name="AreaOfOperatingLease" type="num:areaItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="slno_LesseeOperatingLeaseMonthAndYearOfExpiration" name="LesseeOperatingLeaseMonthAndYearOfExpiration" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="slno_LesseeOperatingLeaseCommencementMonthAndYear" name="LesseeOperatingLeaseCommencementMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
</xsd:schema>
