DbArithmeticExpression arguments must have a numeric common type

Arithmetic with DateTime is not supported in Entity Framework 6 and earlier. You have to use DbFunctions*. So, for the first part of your statement, something like:

var sleeps = context.Sleeps(o =>
    DbFunctions.DiffHours(o.ClientDateTimeStamp, clientDateTime) < 24);

Note that the DiffHours method accepts Nullable<DateTime>.

Entity Framwork core (when used with Sql Server, maybe other db providers) supports the DateTime AddXxx functions (like AddHours). They're translated into DATEADD in SQL.

_{*EntityFunctions prior to Entity Framework version 6.}

I know that this is an old question but in your specific case instead of using DBFunctions as suggested by @GertArnold , couldn't you just invert the operation move out the arithmetic in question from the Lambda?

After all clientDateTime and time24 are fix values so their difference does not need to be recalculated in every iteration.

Like:

TimeSpan time24 = new TimeSpan(24, 0, 0);
TimeSpan time18 = new TimeSpan(18, 0, 0);    

var clientdtminus24 = clientDateTime - time24;

// first get today's sleeping hours
List<Model.Sleep> sleeps = context.Sleeps.Where(
    o => (clientdtminus24 < o.ClientDateTimeStamp) && 
          o.ClientDateTimeStamp.TimeOfDay > time18 && 
          clientDateTime.TimeOfDay < time18 && 
          o.UserID == userid).ToList();

This refactor is usually possible if you are trying to compare the stored datetime shifted by a fix timestamp with an other datetime.