Why does one feel asleep sometimes despite a full rest?

Factual Questions
1

This has bothered me for a while. Why do I sometimes feel like I’m tired even after 8 full hours of sleep?

2

Any number of reasons:

bad mattress
sleep apnea
flopping around all night
no delta sleep

3

If it’s sleep apnea, you may be asleep for eight hours but it’s not efficient sleep in terms of the amount of oxygen you are breathing. This is because the airway collapses and less air gets through. Or you may stop breathing multiple times during your sleep which disrupts rest even though you never know about it.

4

Or maybe you have narcolepsy. Narcoleptics can sleep for hours and hours and not feel rested, since narcolepsy means too much REM sleep and not enough NREM sleep.