Try this:
Select Top 1000000 tblassets.AssetID,
tblassets.AssetName,
Max(Case
When tblUptime.EventType = 3 Then tblUptime.EventTime
Else Null
End) As start_sleep,
Max(Case
When tblUptime.EventType = 4 Then tblUptime.EventTime
Else Null
End) As end_sleep,
DateDiff(MINUTE, Max(Case
When tblUptime.EventType = 3 Then tblUptime.EventTime
Else Null
End), Max(Case
When tblUptime.EventType = 4 Then tblUptime.EventTime
Else Null
End)) As [Minutes Asleep]
From tblassets
Inner Join tblUptime On tblassets.AssetID = tblUptime.AssetId
Where tblUptime.EventType In (3, 4)
Group By tblassets.AssetID,
tblassets.AssetName