How come it feels different to wake up after sleeping late?

On most nights, I sleep at around 12 and wake up at 8. That gives me the requisite 8 hours.

On other nights, I might sleep as late as 2 and wake up at 10. I am still getting my 8 hours, but I feel very groggy and tired when I wake up. I feel this grogginess even if I have started a pattern of sleeping at 2 and waking at 10.

Any reasons why this might be? (I am not a medical person, so please explain any terminology you might use. Thanks!)