What is your overall fitness level like? Age? You said you recently started riding. You're doing 2 or 3 hours of riding which is a fairly long ride/exercise.
For a very long I've felt tired or sleepy after vigorous exercise. I do some bike commuting (morning and afternoon/evening) which is fine. However, I prefer to do vigorous exercise rides in the evenings (make sure you have good visibility clothing and lights if you get out too late).
In some cases,I can roll off the bicycle and into bed within an hour.
In your case, if you were doing well with 20 mile morning rides, then go ahead and keep that up. Perhaps slowly increment them longer. Anything longer, plan on afternoon, evening rides, or longer day rides.