Taking a short half-life drug(venlafaxine) at bed time does waste it?i think 150mg at morning is better than 75mg morning&bed time.

Not wasted: The effects of antidepressants don't depend on their elimination half-lives. Long term changes occur in cells as a result of ongoing exposure to the med. Half-life issues have more to do with immediate side-effects. That said, venlafaxine (effexor) is usually prescribed in time-release versions ("xr") now, to spread absorption over the day. In this case either dosing approach is similar.