If you’re thinking about taking a nap, the shorter the better, reports CBC.

Research suggests if you sleep for more than 25 minutes, you risk falling into the REM state. And when you wake up, you have sleep inertia, where “you’re still kind of asleep and a lit bit kind of spacey.”

Sleep intertia can take about a half an hour to wear off after you wake up from a deep sleep, research suggests.

If you really want to be rejuvenated, drink a cup of coffee, then have a 10-minute nap. Since it takes about half an hour for the caffeine to take effect, you can nap while you wait.