My doctor told me it had something to do with not getting proper REM sleep. One way to tell you're not getting proper REM sleep is if you wake up and remember the dream you were having vividly. That's what I understood him to say.
The nights I take Ambien extra strength, Xanax and Flexeril, I might sleep as much as six hours and feel ok. I dint recommend that combo but I was a little desperate and doctor threw me a bone.
When I wake up during a Vivid dream, I know im going to be tired When I don't, I don't feel so bad