What makes our body decide, ok now it’s time to get up? Why do things like nightmares often allow us to shortcut sleep and wake up early?
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Our circadian rhythm dictates our sleep/wake cycle. And it’s primarily driven by light and temperature. Once the sun goes down, our bodies start releasing melatonin, preparing us for sleep hours before we’re actually ready to go down. Cooler night temperatures also aid in this.
As the sun rises and temperatures increase, our bodies stop producing melatonin, allowing our bodies to wake naturally and slowly over time. Even with your eyes close, you’re still receiving light through your eyelids.
It’s another reason people say if you want to fix your circadian cycle you should go camping for a few days and get away from all the artificial light in your life.
Your body is still able to receive external input while asleep, to protect you from danger. Hence why certain noises might wake you in the middle of the night. Adrenalin helps get your body going in stressful situations while awake and also asleep. However, at night, you’ve still got melatonin coursing through you, so it’s relatively easy to go back to sleep. If something startles you awake after the sun is up, your body has already slowed or stopped the production of melatonin and it’s harder to get back to sleep or stay asleep.
Ooo I know things! First of all, brains are weird, and sleep is also weird, so it is likely I am only giving you part of the picture. All I am providing is the stuff that I am familiar with (I study stress, not sleep).
You have heard of the hormone, cortisol, that is produced by our bodies in response to stress. Cortisol is generally thought of as a stress hormone, however its functions are mainly metabolic. That is, cortisol breaks down energy stores to make that energy available to use (MOAR POWER). In response to a stressor, say a predator, or maybe just going outside, that extra energy is helpful to keep you alive.
However cortisol naturally follows a circadian rhythm (a cycle that repeats daily). This is completely unrelated to stress. The point in the this cycle when cortisol is at its peak is shortly before waking. This makes sense, as you want to increase the available energy you have so that it is ready to use as soon as you wake up. How well cortisol follows this circadian rhythm will depend on MANY factors that I won’t go in to because I don’t study them, but the short of it is, the more routine you have in you sleeping and waking, the better your body will be at sleeping and waking.
As for nightmares, this comes back to stress! Nightmares are scary and basically wake you up ready for that fight or flight. This will cause a release of cortisol, which will then increase the energy ready to use, and then you have all this energy waiting to be used and suddenly you feel like you don’t need to sleep anymore.
Not exactly answering your question but interesting and related to it
How does noise and pain wake us up?
There is an area of the midbrain, right where the cerebellum connects to the brain, that is called the leminiscus lateralis, this is an area with a lot of tiny grey matter spots and a huge amount of nerves going through, those grey matter spots do some basic interpretation of signals going through… Some of the signals that go through there are the signals coming from the cochlea, the organ we use for hearing and from the pain receptors in the body that come through the nerves in the spine, some of the little grey spots in the leminiscus lateralis connect to a few strands of those nerves and detect what is going through them and regulate sleep activity, heartbeat, blood pressure and alertness accordingly when there is a spike in signals or when there is continuous signals…
When there is a spike in signals coming from the ear, like a loud noise, or coming from the pain receptors, like a sharp pain, these little grey matter spots are responsible for making our heart race, blood pump, face flush, get all sprung up and alert, and if we are asleep, wake up almost instantly! And when there is a lot of low intensity signals like ambient noise or diffuse/slow pain (like backpain) it doesn’t reach the threshold for waking us up, but it still increases our alertness, leading to a lighter less profound sleep that is less efficient, leaving us feeling poorly rested and tense after sleeping with pain or noise…
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Everyone gave you a bunch of great answers, so I’ll avoid talking about circadian rhythms and cortisol and melatonin and other things that factor into your sleep schedule, and focus instead on nightmares, specifically ones that tend to wake you up.
Falling is a very common one. When slipping between stages of sleep, sometimes your body has involuntary twitches, and they can be fairly dramatic. We call these hypnic jerks, and it’s thought that the motion causes the sensation of suddenly falling, which translates into your dream. It can be enough to suddenly wake up. Dream interpretation theory says that dreams of falling without stopping or waking up can also be indicative of feeling a lack of control in your life.
Running away from something and suddenly being caught or confronted is also another common waking nightmare, although that one is more straightforward. When dreaming, your brain still ‘responds’ to what you’re perceiving, so if you’re startled in a dream, you’re actually being physically startled, which triggers higher levels of alertness.
Dying is an interesting one. If you die in a dream, you basically always wake up, even if it was a peaceful dream-death. My theory is that our brains don’t know how to simulate death, so when it perceives “dying” in a dream, it sort of freaks out and starts pumping cortisol, which wakes you up.
Any random occurrences of waking up due to a sudden loud noise in your dream was probably a translated noise that actually happened and woke you up, like thunder, a loud car, a slamming door, etc.
Obviously there are more reasons why you might wake up suddenly due to a nightmare, but those are the most common ones that I can think of.
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