1. You wake up in the wrong sleep phase
Your sleep consists of different phases. If you wake up in a non-REM phase, you are tired because it is a state of very deep sleep.
2. Your sleep quality is poor
You sleep with a pet, drink a lot of caffeinated drinks late in the day or there is noise in the background.
If your sleep quality is bad, it does not matter if you sleep short or long. You will be tired when you wake up.
3. You may have a sleep disorder or have medical hangover
Sleep apnea and depression are some of the diseases that can ruin your sleep and make you groggy when you wake up.
Certain types of medicine can also provide a form of hangover. If you suspect that the medicine is ruining your sleep, talk to your doctor.