Ever wake up after a seemingly great night of sleep—say 10 hours—and still feel exhausted? It’s actually a very common problem. Most people arbitrarily set their alarm for when they need to wake up, but you should really set it according to when you body wants to wake up. It’s easier than it sounds. You see, grogginess and feeling refreshed isn’t necessarily caused by how many hours you sleep, but instead by the number of complete sleep cycles you enjoy, according to research published in Applied Cognitive Studies.
When you sleep, you go through five different cycles, with the final phase being REM sleep (the period when dreams occur). During phase one, your vital signs are closest to being awake. During stage four, you’re in your deepest sleep, with your heart rate and blood pressure dropping by as much as 30 percent. Each five-phase sleep cycle lasts about 90 minutes.
So what happens when you wake up during your deep sleep? It’s probably how you feel every Monday morning—exhausted and like you can’t concentrate. This is known as sleep inertia, and a study published in the Journal of the American Medical Association found that morning grogginess could be a bigger impairment than not sleeping at all. (Not that we need to tell you; coffee is popular for reason.)
Your solution: Time your sleep so that you don’t wake up during the wrong phase. A good rule of thumb is to aim for seven-and-a-half or nine hours of sleep per night. If you must sleep less, sleeping six hours might be more restful than seven because you’re more likely to wake up in the first phase of sleep as opposed to a jarring alarm in the middle of your REM sleep.