Have you ever wondered what causes that sleepy feeling that you get in the evening leading up to bedtime? You feel tired, your body feels heavy, and sometimes it even feels challenging to keep your eyelids open. That sleepy feeling is caused by a biological mechanism that drives your desire to sleep, called sleep pressure. We’re going to cover what causes that sleepy feeling, called sleep pressure, why it’s important and what can disrupt this important sleep signal.
Sleep pressure, which is sometimes called sleep drive, is a biological mechanism your body uses to regulate your desire and need for sleep and you experience it as a sleepy feeling. Sleep pressure rises throughout the day as a molecule called adenosine builds up and binds to adenosine receptors that are located in your brain. When these adenosine receptors are saturated with adenosine molecules, you have a high amount of sleep pressure which makes you feel sleepy.
During sleep, adenosine is cleared from your brain. So when you wake up in the morning, if you slept well, the adenosine has been cleared from the adenosine receptors in your brain and you feel awake. In other words, sleep decreases, or resets, your sleep pressure by clearing away the adenosine.
During the day, as energy molecules are burned, adenosine slowly builds up and binds to the adenosine receptors in the brain, slowly increasing sleep pressure again.
Sleep pressure is important because without it, we would struggle to fall asleep at night. It’s what gives us the signal, that feeling or sensation, that it’s time to get some rest. Sleep pressure influences something called sleep latency, which is the technical term for the amount of time it takes you to fall asleep. Without enough sleep pressure, it takes you longer to fall asleep, which means you have a high sleep latency.
A normal sleep latency, or the amount of time it takes to fall asleep, is about 10 to 20 minutes. A high or a low sleep latency can be an indicator of sleep problems. If sleep latency is low, it can be an indicator that you may be sleep deprived. Let’s say you have a night where you only got 6 hours of sleep, instead of the recommended 7 to 8 hours. The following night, it’s likely that your sleep pressure will be high and that will result in a lower sleep latency.
One potential cause of a high sleep latency, as we just discussed, is low sleep pressure…in other words, a potential reason it takes you longer to fall asleep is due to a reduced feeling of sleepiness, or a lower sleep drive. Let’s talk about what can cause a decrease in sleep pressure.
The caffeine molecule is very similar in structure to the adenosine molecule. Because it’s so similar, it can attach to the adenosine receptors in the brain. Now, when caffeine molecules attach to adenosine receptors, it blocks the adenosine molecules from binding to them. That results in a sensation of feeling more awake. Why? Because you’re blocking sleep pressure. When caffeine molecules bind to the adenosine receptors instead of adenosine molecules it prevents sleep pressure from building.
Sleep experts recommend you stop consuming caffeine by noon. This is because caffeine takes a long time to break down and eliminate from the body. Caffeine has an average half life of about 5 hours. This means that it takes 5 hours for your body to eliminate half of the caffeine that you consumed. If we say it another way, 5 hours after you’ve consumed caffeine, half of it is still in your body. And 5 hours is just the average half life, some people metabolize caffeine more quickly and some more slowly, so your body may take less or more time to clear caffeine.
One study found that even 6 hours after consuming 400 mg of caffeine, sleep was reduced by more than an hour. For reference, 400 mg is about the equivalent of a venti, or a 20 ounce cup of Starbucks Pike’s Place roast.
So, if you drink caffeine in the afternoon or evening, it can block your adenosine receptors and prevent sleep pressure from building, and then when it’s time to go to sleep, you end up staring at the ceiling unable to fall asleep. So, caffeine results in a high sleep latency, or a longer time to fall asleep, because it essentially blocks your sleep pressure signal.
Another factor that can impact sleep pressure is napping too close to bedtime. An afternoon nap can be a great way to rejuvenate your mind and body. A 10 to 30 minute nap is a great way to increase daytime wakefulness, improve learning ability, and enhance performance! However, something to be aware of is that since sleeping decreases sleep pressure, taking a nap too close to bedtime can decrease your sleep pressure, which will make it more difficult to fall asleep.
We’ve talked about what decreases sleep pressure, but you might be wondering if there’s anything you can do to increase sleep pressure and improve your ability to fall asleep at night. There is. Exercise, or being very active and increasing energy expenditure throughout the day means you’re burning more energy molecules which causes an increase in adenosine. So exercise results in more adenosine which is going to increase sleep drive and help you feel more tired at bedtime.
Sleep pressure drives the need, or desire, for sleep. It is one factor regulating your sleep-wake patterns and serves as a very helpful cue to you that you are in need of sleep.
Sleep is after all when your brain and body heals and repairs. It’s when the massively important activities of memory consolidation, emotional regulation, growth hormone release, and when so many other healing processes happen. Prioritizing optimal sleep is absolutely critical for maintaining health, and I would even say it’s the most impactful health promoting activity you can do.
Being aware of the things that impact your sleep pressure, like consuming caffeine too late in the day or taking naps too close to bedtime, is key to maintaining healthy sleep pressure, which helps you fall asleep at night. So, pay attention to your body’s cue, in the form of sleep pressure, that it’s time to rest, heal, and restore your body and mind with a night of great sleep.
High stress levels and chronic stress, as well as circadian rhythm disorders are factors that can impact your ability to sleep well.
If you are experiencing chronic stress or abnormally high levels of stress, this article covers mind-body therapies to help reduce stress:
Circadian rhythm disorders can make it difficult to get adequate amounts of quality sleep. This article covers how these disorders can impact brain and neurological health and natural therapies that can help: