It’s a cruel quirk of nature: No matter how many times you hit snooze, you’re still rubbing your eyes and yawning an hour after your alarm goes off. If sleeping is meant to be restful, why do we feel so damn tired when we wake up?
The answer was revealed in this short excerpt from an in-depth interview with sleep scientist Daniel Gartenberg. Read the full interview—including why 8.5 hours of sleep is the new eight hours, the genes that dictate if you’re a morning person or a night owl, and how sleep deprivation can be a tool to fight depression—here.
Quartz: Even if you’ve got enough sleep, why do you still feel tired when you wake up?
Gartenberg: When you wake up, you have something called “sleep inertia.” It can last for as long as two hours. That’s why you get that groggy feeling, and if you’re sleep deprived, it’s going to be worse, too. Studies also show that if you wake up while in deep sleep, you’re going to have worse sleep inertia.
What’s the science behind sleep inertia?
The causal mechanism is a lack of cerebral blood flow when you wake up. It takes a while for the brain to kick back into gear after you are asleep. This “kicking back into gear” is represented by a gradual increase in your cerebral blood flow to normal levels. It starts with the more primitive/ancient parts of the brain, like the brainstem and thalmus, and then spreads to anterior cortical regions after 15 minutes or so. The ability to perform basic cognitive tasks is impacted by cerebral blood flow in these regions, as has been shown in transcrannial doppler somnography studies. This paper gets into the deep science.
Shifts in the blood flow of the prefrontal cortex also suggest that there is a reestablishment of our consciousness going on as well, where we are basically going from a state of forgetting who/what/when we are to our pre-frontal cortex reestablishing our personality and sense of being. I’m sure most people can relate to that feeling of waking up and not really knowing who you are.
What can make sleep inertia worse?
When you wake up in deep sleep, you often feel more tired: It’s like when you wake up to catch that flight before dawn and you feel like you have no idea where you are. If you wake up at the wrong time of a nap, you also feel that way, because the first sleep cycle you go through is very rich in deep sleep, and you’re probably waking up in the middle of that.
If you sleep a healthy amount—ideally 8.5 hours—you’re getting almost no deep sleep by the end of your sleep, as the amount of deep sleep reduces over the course of the night. That means you’re less likely to wake up in deep sleep if you’re well rested, and therefore less likely to feel groggy.
What’s the best way to wake up to avoid sleep inertia?
Instead of trying to time when you wake up so you’re not in deep sleep, it’s usually better to just sleep more. The right way to wake up is very gradually. Both iPhones and my sleep app have this function: You set the time for when you want to wake up, but when the alarm goes off, it starts almost imperceptibly and then ramps up over a 10-minute period. If you didn’t have a good night’s sleep, it’ll take longer for you to wake up: When I’m really sleep deprived, I’ll wake up by minute eight, whereas if I’m not, I’ll wake up right when the thing goes off. We have to shake the snooze thing.
What do you have against snooze buttons?
So here’s the thing: Generally, it’s bad. I understand you have to do it. But what snooze means is that you’re sleep depriving yourself—you shouldn’t have to snooze. You should wake up when you should wake up.
Read next: Why eight hours a night isn’t enough, according to a leading sleep scientist
This article is part of Quartz Ideas, our home for bold arguments and big thinkers.