Sleep is essential for our well-being and health. It is a natural and basic function important for all organisms, from microorganisms to plants to humans, independent of gender, ethnicity, age, or culture. Even many years ago, Greeks and Romans worshiped gods associated with sleep, like Hypnos and his Roman equivalent, Somnus. Poor sleep is for humans what Kryptonite is to Superman: it weakens us. It is not by chance that, on average, one third of our day is spent asleep.

How Do Our Bodies Know When to Sleep?

Sleep is a fundamental biological function, highly regulated by (1) circadian rhythms, which are 24 h cycles that regulate the internal processes of our bodies, and (2) sleep homeostasis, which is the “battery” that regulates our energy levels and tells us when to sleep and when to awaken.

Circadian rhythms are cycles of approximately 24 h (from the Latin circa — “about” — and diem — “a day”) that underlie all the biological and behavioral functions of our bodies. Each cell of the body acts as a biological clock, meaning that we all have multiple clocks inside our bodies, in all tissues and organs. These biological clocks organize the day into 24 h, assigning specific functions, such as sleeping, to specific times of the day [1]. The timing of these clocks is set by a master clock located in the brain. Biological clocks are also affected by the condition of the body, such as its energy level, and on factors in the external environment, such as the amount of light or oxygen that is present or the amount of physical activity that is being performed. One example of a function regulated by biological clocks is body temperature, which is highest between 4 and 6 p.m. and lowest around 4 a.m. Activity and sleep patterns are also regulated by biological clocks. During the day, we are usually more alert and active: eating, playing, working, moving, etc., while during the night, we are less active and more predisposed to sleep.

When the day starts, we are often full of energy, but this energy decreases as the day goes on and at night we feel more tired, because our bodies need to sleep. During sleep time, the body uses energy to perform maintenance and repair, recharging our batteries (Figure 1). Superman also needs to recharge his powers under the sun. So, we sleep when our biological clocks tell us to, and when our batteries are empty. We wake up when our batteries are fully charged. This cycle repeats every 24 h.

Sleep is Important!

Most people are not aware of the importance of having good sleep habits, and only a few sleep enough hours to recharge their batteries. Sleeping less than what your body needs, which is called sleep deprivation, has severe consequences. In the short term, sleep deprivation can cause irritability, poor memory, concentration problems, headaches, and decreased immune defenses. In the long term, sleep deprivation can lead to an increased likelihood of developing certain diseases, including heart disease, obesity, and brain disorders. This also happens to Superman—his powers diminish when he is not exposed to the sun for a long time. The necessary number of hours of sleep needed to fully recharge our batteries varies from person to person and from age group to age group. The National Sleep Foundation, an American organization created to improve health and well-being through sleep education, recommends a suitable number of hours for each age group (Figure 2) [2].

Respecting the number of hours of sleep that our bodies need is very important. However, it is equally important to understand when we should sleep. Each of us has our own internal rhythm, called a chronotype, defined by our internal clocks. Our chronotype determines when we like to sleep [3]. Two main chronotypes, called morningness (being most active and alert in the mornings) and eveningness (being most active and alert in the evenings), are the two extremes. However, chronotype also changes as we get older. Preteens and many elderly people usually experience morningness, while adolescents commonly experience eveningness. Indeed, many teenagers feel excessively sleepy during the day because they have difficulty falling asleep at night and waking up early in the morning.

What is Sleep Hygiene?

Taking too long to fall asleep, sleep disturbances, and daytime sleepiness are the most obvious signs of poor sleep hygiene. Good sleep hygiene includes habits and practices that help you to have a good night of sleep [4] (Figure 3). For example, going to bed and waking up at regular times. Even in exceptional cases, it is important to not go beyond 2 h from our regular routines, as it may deregulate our internal clocks and lead to difficulties falling asleep or waking up at specific hours. Physical exercise is also important to improve sleep quality. However, exercise acts as a stimulant and should be avoided close to bedtime. Similarly, other stimulants, such as chocolate and caffeinated drinks, keep our brains awake and should also be avoided close to bedtime.

Sleeping conditions should also be considered, such as the bedroom’s temperature (16–18°C or 61–64°C), its comfort, and its lighting. Bright light from lamps, cell phones, and TV screens should be turned off and electronic devices should be left out of the bedroom. Even before going to bed, the excessive use of electronics and bright lights should be avoided. Why? At dusk, our brains start to prepare our bodies to sleep. The absence of intense light allows the production of a chemical signal that makes us feel sleepy. This chemical is called melatonin—the sleep hormone. Melatonin levels start to increase after sunset until the middle of the night (2–3 a.m.). Then, melatonin levels begin to decrease until dawn. When we use electronic devices, the screens’ light tricks the brain into thinking it is daytime again—the blue light emitted by screens is perceived by the brain as sunlight. This phenomenon delays melatonin production and consequently delays the start of sleep, similar to the Metropolis city danger signals that make Clark Kent awaken as Superman. Preparing ourselves for sleep by avoiding stimulating light helps us to train our bodies to sleep better and healthier.

Be Aware of Sleep Disorders

Sleep disorders can cause difficulties with falling asleep or staying asleep, affecting the quantity and quality of our sleep. There are over 80 diagnosed sleep disorders [5] (Table 1). These can be caused by irregular sleep schedules, caffeine or alcohol, drugs, aging, a genetic predisposition, or the presence of other medical conditions, such as anxiety or brain disorders. We can imagine these disorders as Kryptonite, which weakens Superman and puts Metropolis city into serious danger.

If good sleep hygiene does not improve ongoing sleep difficulties, consulting a medical doctor or a sleep specialist should be the next step. First, the medical doctor will talk with the patient and evaluate his/her sleep habits, sleepiness during the day, presence of diseases, and current medication. If bad sleeping habits are detected, the doctor will advise the patient to change those habits to promote better sleep hygiene. If bad sleeping habits are not the problem, the patient must be further evaluated. Sleep might be monitored at home or at a Sleep Unit, using electronic devices. If a sleep disorder is diagnosed, there are several treatments that might be followed depending on the diagnosis. For example, a medical doctor might prescribe medication containing melatonin, if indicated. The status of the patient will be monitored by the Sleep Unit throughout the treatment and, depending on how the patient does, medication can be adjusted or removed. Other treatments might involve wearing devices during sleep, such as masks that help breathing while sleeping, or a psychologist to help people to sleep better.

To finalize, we recommend you to see this video where you can review all about sleep!

Video—Animation on the importance of sleep, developed together with the Portuguese Sleep Association.

Sleep Well, Sleep Tight

Now that you know how, when and why our bodies need to sleep, habits and practices that help to sleep better or that, on the contrary, disturb a good sleep night, existent sleep disorders and what to do in those contexts, there are no excuses to not have a good night of sleep! Do not let a bad sleep night be your kryptonite, superhero!

Glossary

Circadian Rhythms: ↑ 24 h rhythms, regulated by biological clocks. For example, we go to sleep every 24 h, so sleep/wake cycles have a circadian rhythm.

Sleep Homeostasis: ↑ Body mechanisms that control our need to sleep. It can be compared to a battery - when it is fully charged, we have more energy to carry out daily tasks; when it is empty, we have low energy and need to sleep to recharge the battery.

Biological Clocks: ↑ Internal clocks that regulate rhythmic cycles, like that of sleeping and waking.

Chronotype: ↑ Inner time preferences, set by our internal clocks, that determine when we are alert and productive during the day, either in the early morning hours or later in the day.

Sleep Hygiene: ↑ Habits that lead to the right amount of sleep and good quality sleep.

Melatonin: ↑ Called the sleep hormone; a chemical signal produced by the brain in response to the absence of light that makes us feel sleepy.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.