I think this a common thing that occurs one time or another to just about all of us. It is not always due to depression, anemia, medication,boredom or any other condition, though they too can cause this.
Sometimes, we just are tired, and if we happen to take a nap and awake, feeling it was the am or pm, (disoriented)that's normal too.
Students and non students, having a busy day can make us just plain tired.
The best kind of "tired" I like but haven't had for awhile (less active ) is when I've worked physically hard outdoors in the fall, or when I have taken some nice brisk walks. I do find with exercise the tiredness hits somewhat delayed, due to that short spurt of energy that comes right after exercise.
If it does seem to be bothersome or starts to upset your daily functioning/lifestyle, then it may be something of concern, and should be taken up with your doc, may be time for a check up.
Gee, talking about this is making me feel a need for nap
Well, I didn't sleep well last nite, maybe 2-3 hours, had to get up early, so yeah, I guess a nap may help at the present.
Take care y'all,
DE