Sleep in particular occurs in cycles between deeper and lighter sleep, with different types of brainwaves at different points. For most people, these cycles last roughly 90 minutes.
If you wake in the middle of a cycle, you’ll feel groggy. Waking at the end of a cycle is easy, and allows you to feel alert even if you haven't had enough sleep yet to feel refreshed.
If you wake in the middle of a cycle, you’ll feel groggy. Waking at the end of a cycle is easy, and allows you to feel alert even if you haven't had enough sleep yet to feel refreshed.