Our sleep works in 90-minute chunks. These chunks contain a phase of light sleep, deep sleep, REM sleep, then a second light phase. However, when awaking, you usually interrupt the cycle during REM, that's why you'll frequently wake up at the end of a dream.

You can utilise this to sleep for exactly the right amount of time to feel fully refreshed in the morning, as waking up at the end of a cycle will leave you feeling the best. This post is basically stolen from this video, but there's a good way to use this knowledge.

Often people find themselves waking earlier than they intend to. If you wake up early and want to sleep in a little more, the ideal time to wake yourself up is in 90 minutes, as that's the end of the cycle. This way, you'll be able to get out of bed much easier than if you only slept for a shorter amount of time, especially in the 20-30 minute ballpark (deep sleep.)