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Sleep Health

Why Do We Need Sleep? What Happens When We Sleep

    Last Updated
    June 7, 2023
    min read

    We sleep for nearly a third of our lifetime, but few understand why we sleep and why sleeping a minimum number of hours is important. It’s clear that if you get enough sleep, you feel refreshed and alert, but getting sufficient hours of high-quality sleep has an array of hidden benefits on your body.

    If you’ve ever wondered “Why do humans need sleep?”, here are several health reasons why we sleep – from improving our emotional well-being to boosting cellular restoration. We'll also explore the consequences of not getting enough sleep, including the negative impact on our mood, cognitive function, and overall health.

    Why Do We Sleep, Anyway?

    reasons we sleep

    We sleep because getting enough rest helps us feel refreshed and energized for the day ahead. However, there are unseen health benefits to achieving a sufficient number of hours of high-quality sleep. 

    Here are some of the biggest answers to the question, “Why do we need sleep?”:

    Brain Function

    Sleep is important to proper brain function. When you sleep, your brain’s glymphatic system is responsible for clearing toxic waste that has built up in your brain cells throughout the day. This allows your brain to function properly when you’re awake, helping you feel alert and focused.

    Studies have found that sleep affects many different features of brain function, such as:

    Energy Conservation

    Energy conservation theory is one of the more popular ideas that may explain why we sleep. This sleep research theory states that sleep is necessary for conserving energy during parts of the day and night that historically tended to be the least efficient times to hunt for food.

    By allowing your body to work at a lower metabolism, sleeping lets you reduce the calories needed to function – his idea is supported by findings that our metabolism slows during sleep. Getting at least 8 hours of sleep can save your body much-needed energy.

    Cellular Restoration

    Another major theory that may explain why we sleep is the restorative theory, which states that our bodies need sleep to repair themselves. Studies have shown that our bodies release growth hormones during sleep, strengthening our immune systems and repairing our damaged tissue. 

    Sleep has also been found to stimulate the production of proteins that promote cell growth and division, two functions that are essential to tissue repair, a strong immune system, and cell maintenance.

    Emotional Well-Being

    Another explanation for why we need to get enough sleep is for the maintenance of  emotional well-being. During sleep, brain activity increases in areas of the brain that regulate  emotions, supporting healthy brain functions. Sleep deprivation, on the other hand, has been linked to a higher risk of aggression, depression, and anxiety.

    Weight Maintenance

    Sleep affects hormones connected to hunger and weight management. 

    When you’re asleep, levels of ghrelin, the hormone that sparks appetite, decrease. On the other hand, sleep deprivation increases your ghrelin levels while reducing leptin production, the hormone that causes satiation after eating.

    This means that lack of sleep causes a hormone imbalance that can make you feel hungrier. 

    Additionally, researchers found that sleep deprivation is connected to a higher risk of:

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    How Much Sleep Do We Need?

    How much sleep someone needs will vary from person to person:

    Age group

    Age range

    Recommended hours of sleep every day


    0-3 months

    14 to 17 hours


    4-11 months

    12 to 15 hours


    1-2 years

    11 to 14 hours


    3-5 years

    10 to 13 hours

    School-aged children

    6-12 years

    9 to 11 hours


    13-17 years

    8 to 10 hours


    18-64 years

    7 to 9 hours

    Older adults

    65+ years

    7 to 8 hours


    Note that these sleep duration recommendations may also be affected by your overall health and certain lifestyle factors. For example, people who are very physically active or have physically demanding jobs may need to get more sleep. On the other hand, people who suffer from sleep conditions such as insomnia or sleep apnea may need to consult their doctor to learn how much sleep they need.

    Apart from getting enough rest, make it a point to prioritize healthy sleep habits to improve the speed and ease in which you fall asleep. Read our guide on what to do when you can’t sleep for tips to fix your sleep schedule and help you fall asleep quickly.

    What Happens When We Don't Get Enough Sleep?

    When we don’t get enough sleep, it negatively affects our physical health, mood, and cognitive performance. Here are some of the most common consequences of not getting enough sleep:

    • Mood swings and irritability: A lack of sleep has been found to decrease positive moods and instead increase negative mood states, such as irritability, sadness, and anger. This may affect abilities to manage your stress and your relationships with other people.
    • Increased risk of workplace injuries and accidents: Sleep-deprived people are more likely to get into car accidents, make mistakes on the job, and cause workplace injuries.
    • Difficulty focusing: One of the symptoms of getting insufficient sleep is increased feelings of exhaustion. Feeling tired may cause you to have trouble paying attention to tasks.
    • Memory problems: Scientists have found that sleep deprivation affects both short and long-term memory, impairing your ability to remember and process things. 

    Why Do We Sleep FAQs

    Where does the mind go when we sleep?

    It’s still not fully understood where the mind goes during sleep. The contents of  thoughts during sleep and dreaming as well as how we process the outside world during sleep are still being studied. 

    During sleep, it’s known that you cycle through four stages: non-REM (rapid eye movement) sleep where your body enters light sleep, light sleep, deep sleep, and REM sleep.

    Each stage is critical for different body functions, such as processing information, replenishing your energy, and repairing cells and muscles. Learn more about sleep patterns, brain waves, and what happens in each stage in our guide to the different stages of sleep.

    Why do we sleep at night and not day?

    It’s generally suggested by studies that we sleep at night and not during the day because of the effect of light on our circadian rhythms

    Your circadian rhythm is your body clock, which regulates your sleeping and waking cycle. A region known in your brain as the hypothalamus processes certain signals when your eyes see natural or artificial light. These signals help your brain establish if it’s daytime or nighttime.

    When natural light fades and evening comes, your body starts releasing melatonin, the sleep hormone that causes drowsiness and signals the body it’s time to rest. During morning sunlight, your body releases cortisol, a hormone that makes you more alert and energetic. Because of this biological cycle, we tend to sleep at night instead of during the day.


    About the authors

    April Seifert

    April has nearly a decade of writing experience, with 3+ years specializing in sleep content. Her work has been covered by sites like BuzzFeed, Entrepreneur, and Money Under 30. She has an M.A. in Magazine, Newspaper, and Online Journalism from Syracuse University and a B.A. in Communication Studies. As a self-proclaimed expert in all things cozy, she sleeps better at night, knowing her writing helps others sleep in comfort, too.