How Sleep Works
Read on to learn about how sleep works, what happens in your brain and body during sleep, and how it evolved.
Read on to learn about how sleep works, what happens in your brain and body during sleep, and how it evolved.
Sleeping is such an essential part of our lives, and we spend as much as one-third of our time doing it. But even though we’ve spent decades researching sleep, we still can’t say why we do it. One thing is for sure, we can’t go on without sleep, and it has undeniable benefits to our health.
A lot of people think about sleep as a time when your body and brain are shut down, but it’s far from it, our brain is working at night, repairing everything and preparing for the next wake period. The nightly rest is essential for the restoration of our minds, and the maintenance of our bodies.
Lack of sleep is associated with many sleep disorders and can lead to many diseases like heart disease, obesity, high blood pressure, and diabetes. It also affects your memory, mood, concentration, immune system and your overall health in general.
Read on to learn about how sleep works, what happens in your brain and body during sleep, and how it evolved.
There are several different brain structures involved in sleep. The hypothalamus is located deeply in the brain, and it contains groups of nerve cells that control sleep and arousal. The suprachiasmatic nucleus (SCN) is situated within the hypothalamus, and it is made out of thousands of cells that receive information about light exposure from your eyes. SCN then uses this information to synchronize your circadian rhythm with the light/night cycle. Most blind people can detect light exposure, so their brains can still adjust their sleep/wake cycle.
The brain stem is communicating with the hypothalamus to control the transition between sleep and wake. Along with hypothalamus, brain stem produces the chemical called GABA that reduces the arousal centers in hypothalamus and brain stem. Pons and medulla are parts of the brain stem, and they play a vital role in REM sleep, relaxing muscles in charge of body movement. That way, we won’t move and act out our dreams.
The thalamus gets the information from our senses and then sends it to the cerebral cortex, a part of your brain in charge of interpreting and processing information. During most sleep stages, the thalamus is inactive so you can tune out the outside world. During REM sleep, the thalamus is active, and it is sending images, sound, smells and other sensations; and that is when you dream.
The pineal gland receives signals from SCN and increases the production of melatonin, which is the vital hormone for regulating sleep.
The amygdala is a structure necessary for processing emotions, and it is active during REM sleep. That is why dreams can help you process your feelings better.
There are two types of sleep: rapid eye movement (REM) and non-REM (NREM) sleep. NREM sleep consists of three stages, and you usually cycle through all of these stages three to five times during the night.
Stage 1 of non-REM sleep is a transition between being awake and falling asleep. It only lasts several minutes, during which your breathing, heart rate, and eye movements slow, and your muscles relax and sometimes twitch.
Stage 2 of non-REM sleep is a phase of light sleep that happens before you enter deep sleep. Your breathing and heart rate slow even more, and your muscles continue relaxing further. Your eye movement stops, and your body temperature drops. Your brain shows slower wave activity accompanied by occasional electrical bursts.
Stage 3 of non-REM sleep is a phase of deep sleep crucial for the restoration of your body and feeling refreshed in the morning. This deep sleep is longer during the first half of the night. Your heart and breathing rate are at their slowest in this stage. Hormones are released, such as growth hormone, essential for growth and development. The brain waves slow even more, your muscles are relaxed, and it may be difficult to wake you up. If awakening happens during this stage, you’ll usually experience the unwanted grogginess.
REM sleep occurs every 90 minutes. Your eyelids are closed, but your eyes move rapidly from side to side. Your breathing rate becomes faster and irregular, and your heart rate increases along with blood pressure, and they are almost at the conscious level. Your brain waves signals are mixed, and they resemble the conscious ones as well. Most dreams occur during the REM stage. That’s why your brain shuts down your limbs movement, so you don’t act out during your dreams. Older people tend to get less REM sleep than younger people.
Sleeping is a pretty wide phenomenon, and it is found everywhere in the animal kingdom, from fruit flies to reptiles, frogs, birds, and mammals. When you think about nature and sleep, it just seems like it isn’t a good idea. Spending long periods motionless and with lower senses toward external stimuli can make you an easier target for many predators lurking out there. So why did natural selection go towards supporting sleep?
Sleeping helps rejuvenate the brain and the body, and it helps conserve energy. Predators who would hunt their prey 24 hours a day, would destroy thoroughly pray populations, and then they would die off as well, as they wouldn’t have anything to eat. On the other hand, species with natural predators have a smaller chance to run into a predator while they are hidden and resting, compared to being and the open and looking for water, food or a mate.
While sleep is found in both vertebrates and invertebrates, there are no sleep signs in unicellular organisms. And sleep patterns vary a lot through animal kingdoms. But if you compare brain waves in some animal with a much simpler brain structure, they would be similar to those to humans and other animals with more complex brains. REM sleep is only found in mammals and birds, while NREM sleep is found amongst others.
It’s interesting that animals can feel sleep deprivation as well. When fruit flies were made to stay awake during the time they were supposed to be asleep, they tried to make that up by sleeping the next chance they got, just like we do.
Some birds and sea mammals can sleep with only one half of their brain. This process is called uni-hemispheric sleep and can last up to two hours. That allows them to rest while still staying alert in case some predators comes nearby.
But why have we evolved to sleep in one long interval instead of having a few shorter ones during the whole day? It appears that it is safer to have one long period of sleep, especially during the night, where our primary sense – our vision, is not at it’s best.
Scientists also hypothesize that sleep patterns in our human lineage have also changed to support the development of higher intelligence. Great apes, while still sleeping in the trees, have started making nests, which allowed them a more stable environment for sleep. As we left the trees, the ground has given us even more sleep stability, which allowed us to get more quality sleep. Better rest gave us the opportunity to spend more time socializing and learning, which lead to higher intelligence.
Genetic background may have a significant role in how much sleep we need. Scientists have identified multiple genes involved with sleep and sleep disorders, as well as “clock” genes that affect our circadian rhythms and timing of sleep. A lot of genes in the cerebral cortex change their expression during sleep and wake periods. Genetic models such as worm, zebrafish and fruit fly are helping scientists identify molecular mechanisms and genes responsible for sleep and sleep disorders.
We all know the feeling of moodiness and irritation, once you miss one night of sleep. Even though an occasionally missed sleep won’t cause you long term problems; chronic sleep deprivation is bad for your health. Sleep is essential for maintaining your body and mind.
Sleep improves health. If you are sleep deprived for days, you might start gaining weight and getting sick. It also affects your metabolism and stress levels. These changes can lead to high blood pressure, increased body temperature, weight gain, diabetes, inflammation, and a reduced ability to fight off pathogens and infections.
It also affects learning and memory. REM sleep helps you remember procedures and foreign languages, and it helps with your logic ability, as well as your ability to process emotions. A good night’s sleep will allow you to be more concentrated the following day, and to perform tasks better.
Sleep helps you conserve energy and regenerate body and mind. Some studies even argue that the main reason why we sleep is to let our neurons have the maintenance they need. Without it, our neurons would be under too much pressure all the time, so giving them the required rest and time to do some housekeeping is essential for their excellent functioning.
Since sleeping is so important, here are some tips on how to get the best out of it.
Dusan is a biologist, a science enthusiast and a huge nature lover. He loves to keep up to date with all the new research and write accurate science-based articles. When he’s not writing or reading, you can find him in the kitchen, trying out new delicious recipes; out in the wild, enjoying the nature or sleeping in his bed.