I have that ocassionaly, but not too often

Usually a nonrestful sleep is the result of the lack of REM sleep, rapid eye movement, its the final stage of sleep and its the lightest, its when most dreams happen. The other stages of sleep are for the body, but REM is for the mind, without getting enough of it your mind'll feel super tired and nasty.

Try to get a bit more sleep, maybe take a power nap or two during the day, but dont take more than a 20 minute one or you'll wake up feeling groggy and probably worse than you did when you woke up.