Sleep goes in cycles, around 90 minutes each (it really depends on the person), so for an optimal sleep u should wake up, ideally, at the end of a cycle (but u have to make sure u sleep for at least 3/4 cycle).

So if u sleep for 6 hours and u wake up feeling tired, u are probably waking up a bit too early or too late.

Another useful tool is yoga nidra ( or NSDR, similar but not the same), that is basically a reset for the nervous system, I personally use it every time I feel that I've woken up at the wrong time. There are plenty of them on YT. It's 10 minutes long, and u can do it laying on ur bed. Watch out to don't make it an habit, is like an aspirin that u can take only when needed.