Naps can be a tricky endeavor. You have to balance out that sweet, sweet extra sleep time with not going so long that you, 1) feel worse when you wake up, and 2) don’t screw up your full night sleep.

Thankfully, a sleep expert has come up with the perfect duration for a nap! Dr. Jeffery Iliff says that if you take a 20-30 minute nap, you don’t have time to go into a deep sleep, so you feel refreshed. If you need a little more, go for 90 minutes, as that completes a full sleep cycle.

Here’s the video of him explaining his theory. The full vid is nearly 20 minutes, and there’s some more great sleep advice in there.