The Sleep Science You Need to Know

What Is a Sleep Cycle and What Are Its Stages?

A sleep cycle is a sequence of orderly stages that the body goes through during sleep, lasting about 90 minutes per cycle. On average, a person experiences 4 to 6 cycles per night, repeating in a loop.

Apple Health simplifies sleep cycles into the following four stages:

• Deep Sleep

  This usually occurs in the later part of the sleep cycle and is the deepest stage of sleep. During this phase, the body undergoes recovery and repair, growth hormones are released, and the immune system is strengthened. Deep sleep is crucial for physical and mental restoration, memory consolidation, learning enhancement, and boosting the immune system. It’s an essential phase for maintaining overall health.

• REM Sleep

  REM (Rapid Eye Movement) sleep is the stage where brain activity is most intense. During REM sleep, your eyes move rapidly, brain activity increases, and most dreams occur in this stage. REM sleep is vital for the brain as it aids in memory consolidation, emotional regulation, and the development of creativity. This stage is critical for cognitive function and mental health, making it an indispensable part of the sleep cycle.

• Core Sleep

  Also known as light sleep, this is an early stage of the sleep cycle and typically takes up most of the sleep period. During core sleep, the body further relaxes, and breathing and heart rate become more regular. This stage represents the initial deepening of sleep, helping to alleviate the day’s fatigue and laying the groundwork for deeper sleep stages.

• Awake Time

  This is the transitional phase between active daytime wakefulness and sleep. During this stage, the body is relaxed, but consciousness remains awake. Throughout the night, we may briefly wake up without even realizing it.

Understanding the sleep cycle can help us better manage our sleep schedules and optimize sleep habits. For example, avoiding disruptions during deep sleep can ensure you get adequate rest and recovery.

Related Literature:

• Hirshkowitz M. (2004). Normal human sleep: an overview. The Medical Clinics of North America, 88(3), 551–vii.

What Is Circadian Rhythm and Why Is It Important?

Circadian rhythm, also known as the biological clock, operates on roughly a 24-hour cycle and regulates our sleep-wake cycle along with many other physiological and behavioral processes. Many factors influence circadian rhythm, with light being the most significant. Sunlight is a powerful regulator of our biological clock, especially morning light, which helps set our circadian rhythm. Light from screens and devices can also impact this rhythm.

Circadian rhythm is crucial for our health, affecting not only sleep quality but also cognitive function, emotional regulation, and the immune system. Disruption of the circadian rhythm can lead to sleep disorders like insomnia, affecting next-day alertness and productivity. Long-term circadian disruption may increase the risk of depression, heart disease, and type 2 diabetes.

Understanding circadian rhythm offers important insights for our sleep habits:

• We should aim to go to bed and wake up at the same time every day to maintain rhythm stability.
• Ensure ample sunlight exposure during the day and reduce blue light exposure (such as from screens) in the evening.

Related Literature:

• Czeisler, C. A., & Gooley, J. J. (2007). Sleep and circadian rhythms in humans. Cold Spring Harbor Symposia on Quantitative Biology, 72, 579–597.