Sleep Cycle and Circadian Rhythm

Sleep Cycle and Circadian Rhythm

Dr. Saarangan Krishnakumar

Have you ever wondered what happens at night when you fall asleep? It’s a rollercoaster ride where your brain goes through the ups and downs, leading to rest and recovery of the body. Our daily sleep is regulated by three main processes:

  • Sleep cycle
  • Sleep drive
  • Circadian rhythm


What is a sleep cycle?

Overnight sleep constitutes several hourly sleep cycles with 4 stages. A healthy individual has 4-6 sleep cycles with an average of 80 - 90 minutes each. The first sleep cycle  is usually short with an average span of 60 mins and the latter tend to fall between 90 - 120 minutes. The variation in sleep cycles are dependent on the four stages of sleep and various internal and external factors.



The 4 stages of the sleep cycle consists of 3 stages of non-rapid eye movement (NREM) sleep and 1 rapid eye movement (REM) sleep. The N1 phase is the transition from wakefulness to snoozing, it is important in prolonging the sleep cycle where the person is susceptible to disturbances. This phase tends to minimize in the latter sleep cycles through the night. 

The N2 phase or light sleep, is wherein the muscles relax, eye movements reduce, drop in body temperature and heartbeat takes place. The time of this phase extends to longer periods deeper into the night and this phase constitutes the major portion of daily sleep. 

N3 phase or deep sleep further relaxes the body with reduction in muscle tone and heart rate. This phase of sleep is important for  muscle and bone growth, hormone production and  body recovery. This phase usually is longer during the start of night and transfers to the REM phase through the night.

The REM phase has an increased brain activity with eyes moving even when they are closed and leaving aside the muscles responsible for breathing, all other muscles are in a state of inertia. This phase is responsible for creativity, cognition and dreams as they are more significant and vivid in this phase. During the first phase the REM sleep lasts about 5 to 10 mins but increases in further sleep cycle constituting up to 25% of daily sleep. Sleep architecture is the process of breaking down a person’s various sleep cycles. The sleep monitoring devices in smart watches provide it as a hypnogram (sleep vs time).

What is a sleep drive?

Sleep drive is the internal messenger which signals our brain to sleep (high sleep pressure) after being awake for a prolonged time. It also signals the brain to wake up (low sleep pressure) when our night sleep is complete and there is a rise in alertness. The equilibrium state that balances our sleep drive is called sleep homeostasis. This works in cohesion with circadian rhythm in maintaining the sleep/wake homeostasis.  

What is circadian rhythm?

It is derived from the Latin word “circa diem” which means “about a day”. It can simply be called the internal body clock which helps in maintaining and regulating essential processes such as hormone synthesis, satiety, cell regeneration and sleep/wake in a 24 hour cycle. Circadian rhythms are controlled by the hypothalamus in the brain, but are influenced by  external factors called “zeitgebers” such as day-night cycle, changes in season and exposure to sunlight.

During the day, the light cycle makes the internal body clock generate alertness through the hormone called histamine. On the other hand, with night and less exposure to light leads to melatonin production to induce sleep. The reason why doctors prescribe antihistamines when you are not feeling well, is to inhibit histamine production in the brain and induce sleep. Vice versa consumption of caffeinated drinks inhibit melatonin and aid in staying awake.

What is sleeplessness?

Sleeplessness or Insomnia is the persistent difficulty to fall and remain asleep even though you had the time. These people tend to fall asleep during daytime also impairing their cognition. If the snoozing time in the night is more than 30 mins it is considered insomniac. The current estimates suggest  one-fourth of the world population live with insomnia with the majority of them being over 50 years old. The major cause of sleeplessness is due to  improper cohesion between circadian rhythm and sleep drive. External and internal factors such as: 

Hormonal imbalance

Extended exposure blue light


Lifestyle changes

Anxiety and Depression

Alcohol and drugs

Shift work

Over caffeinated


Living environment (bed, temperature, noise)

 How to maintain good sleep hygiene?

Sleep Hygiene is the set of habits that can be followed to get adequate sleep to improve physical and mental health. Seniors and younger people with sleep disorders can consult physicians for possible corrective measures that can be taken to improve the sleep pattern.

  • Sleep environment
  • Sleep routine
  • Lifestyle changes 

    In general, a sleep environment comprises a comfortable mattress, pillows and a bed which is preferred for sleeping with an ambient room temperature of 24℃.  Reduce  the exposure to light by using an eye mask and loud noise by using earplugs. The use of calming scents and low audible music can create a pleasant environment to relax and doze off. 

    A sleep monitoring device can be a good purchase to understand your daily sleeping pattern and make adjustments to the routine

    With the bed in place, the pre-sleep habits can be a set time to go to bed with 30 mins to relax yourself. Try not to use electronics during that time as the blue light can stimulate your brain and also slow your melatonin production. A set bedtime and wakeup schedule can help to synchronise your sleep pressure with circadian rhythm. The current recommendation for a healthy adult is to have 7-8 hours of sleep. 

    Healthy lifestyle can do a lot of good to improve your sleep hygiene. Sunlight exposure and active exercising can induce sleep at night with synchronisation to circadian rhythm. Quitting smoking as nicotine disrupts sleep is the major reason for several health problems. Reduced consumption of alcohol and caffeine can contribute significantly to the sleep cycle. Allow sufficient time post dinner for the body to digest the food consumed with a routine time to wind down to the bed. 


    About Dr. Saarangan Krishnakumar

    Dr. Krishnakumar is a research scientist with a distinct interest in bio-active phytochemicals and antioxidants. His expertise lies in cellular and molecular biology techniques, analysing and interpreting spectroscopy and confocal microscopy. He has published several papers in the field of tea polyphenols and oxidative radical scavenging in international scientific journal such as The Journal of American Chemical Society (JACS).


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