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Why do humans sleep at night?

Humans have evolved to sleep mostly at night for several important reasons related to health, safety, and circadian rhythms. In this article, we’ll explore the evolutionary and biological factors that explain why human sleep patterns are tied to the night.

Exposure to sunlight and melatonin release

One of the main factors influencing human sleep patterns is our exposure to sunlight. Sunlight exposure helps regulate the release of melatonin, a hormone that makes us feel sleepy. During the day, sunlight exposure suppresses melatonin production. But at night, when it’s dark, melatonin levels begin to rise, making us feel drowsy and ready for bed.

Melatonin plays a key role in regulating our circadian rhythm – our internal 24-hour cycle that influences when we feel awake versus tired. As melatonin levels rise in the evening and remain elevated at night, it induces sleepiness and helps maintain healthy sleep cycles.

The circadian rhythm

The circadian rhythm is our biological clock that regulates the timing of sleep and wake cycles. This internal clock is synchronized to the 24-hour light/dark cycle through exposure to sunlight and melatonin release. At night, melatonin release induces sleepiness in alignment with our innate circadian rhythm.

Disrupting the circadian rhythm by exposure to light at night can make it harder to fall asleep and reduce sleep quality. That’s why avoiding screen time and bright lights before bed can improve sleep – it allows normal melatonin release in sync with our biological clock.

Safety and predator avoidance

In addition to circadian regulation, sleeping at night likely provided an evolutionary advantage for safety and predator avoidance. Being awake and active at night would have made our early human ancestors more vulnerable to predators and other environmental dangers.

Sleeping in shelters and groups offered protection at night. So over thousands of years, natural selection likely favored individuals whose circadian cycles adapted to being asleep when it was dark out.

Other factors influencing nighttime sleep

A few other key factors contribute to why we tend to sleep at night:

  • Cooler nighttime temperatures are more conducive to sleep than hot daytime temperatures.
  • Darkness triggers release of the sleep-promoting hormone melatonin.
  • Nighttime routines help transition our bodies and minds into sleep mode.
  • Being awake all day builds up a sleep debt and tiredness that makes us ready for sleep at night.

The stages of human sleep

When we sleep at night, we go through different sleep stages in cycles throughout the night. Human sleep consists of two main types:

  • REM (rapid eye movement) sleep – dreaming sleep, when brain activity increases and eyes move rapidly behind closed eyelids.
  • Non-REM sleep – deep sleep divided into stages N1, N2, and N3, with stage N3 being the deepest sleep.

We cycle between REM and non-REM sleep throughout the night in patterns tied to circadian rhythms. The deepest non-REM sleep usually occurs in the first half of the night. REM sleep occurs more towards morning.

The benefits of quality nighttime sleep

Getting sufficient deep sleep at night provides many health benefits, including:

  • Restoration of energy levels and improved daytime alertness
  • Memory consolidation and learning
  • Tissue growth and repair
  • Hormone regulation
  • Bolstering immune function

Poor or insufficient nighttime sleep can impair cognition, productivity, and overall health. Adults need 7-9 hours of quality sleep per night for optimal health and wellbeing.

Disrupted sleep patterns and health

While humans are biologically programmed to sleep mostly at night, many factors can disrupt this pattern and affect health:

  • Shift work and night shifts – disrupts circadian rhythms
  • Sleep disorders – conditions like insomnia disrupt sleep quantity and quality
  • Poor sleep habits – inconsistent bedtimes, screen exposure at night
  • Medical conditions and medications – can affect sleep cycles

Disrupted sleep from these factors is associated with increased risks for some chronic diseases. Short and poor quality sleep linked to long-term issues like:

  • High blood pressure
  • Heart disease
  • Stroke
  • Diabetes
  • Obesity
  • Depression

Optimizing sleep quantity and alignment with circadian rhythms can improve health. This highlights the importance of good sleep habits for robust nighttime sleep.


Human sleep patterns evolved to synchronize with nighttime light/dark cycles for optimal health and safety. Exposure to daylight coordinates melatonin release for drowsiness at night. Nighttime sleep provides restorative benefits tied to human physiology and circadian rhythms. Misalignment between circadian cycles and sleep disruptions can negatively impact wellbeing. Understanding the biological factors linking human sleep and nighttime provides insights for improving sleep hygiene in the modern, artificial light environment.