I've faced this a few times.
Most of the time I could feel that they were somewhat related to

1. Sleep cycle changes,
2. Stress
3. Lack of sleep.

(Yeah, sometimes I do get lack of sleep, on some days I even sleep an hour in one day, and then I oversleep for 12-13 hours; its bad I know, but I'm working on it!) during those times only, I felt these things...