I so feel you, I get this too. Sometimes people get enough quantity sleep, but not enough quality. Are you dreaming? If you don't go through all the stages of sleep, then it's not good quality. Secondly, it can be stress-related. Being emotionally exhausted makes me feel physically tired too, and I start falling asleep at work. I especially feel like falling asleep in the afternoons, so I try to take a walk on my break instead, which really helps wake me up. Or I take bathroom breaks, stand while working, run errands to a different office, anything to get away from the monotonous computer task that is putting me to sleep. Humans are made to take afternoon naps - many cultures build them into the work day. It's normal to get a full night's sleep but still want an afternoon nap. This is my experience - I hope it helps.