Circadian Rhythms

Every cell and organ in your body has an internal clock which generates an internal rhythm. For instance, there are 24-hour rhythms for breathing, heart rate, gastric acid, hormonal secretion and even cell rejuvenation. Sleep–wake cycle is the most prominent of these rhythms. These ‘peripheral clocks’ are controlled by the SCN (suprachiasmatic nucleus), your master circadian clock.

RHT: Retinohypothalamic Tract

SCN: Suprachiasmatic Nucleus

Balance between the C (circadian) and H (homeostatic) drives determines when you sleep and wake-up in the 24- hour period. C drive depends on the SCN, which in turn is entrained by the light-dark cycle, while the H drive depends on how long you have been awake.

C drive: When light enters the eye, it stimulates melanopsin containing ganglion cells of the retina transmitting electrical signals through RHT to the SCN, increasing alertness.  At the same time, melatonin production by the pineal gland (sleep modulating hormone) is supressed. Similarly, when the sun goes down, SCN activity is reduced and melatonin production is suppressed, resulting in reduced alertness. This sends a signal to the brain that it is time to sleep.

Therefore, when the sun rises, your alertness will increase and when the sun sets, you will slowly become sleepy, so that you can fall asleep a few hours later.

H drive increases when your body and mind are active. It is low when you wake up in the morning and increase slowly though the day., as a result of accumulative brain activity. It reaches its maximum level at bedtime.

Table 3. Circadian and Homeostatic influence on sleep and wake.

C: circadian drive, H: homeostatic drive

Both C and H drives are low when you wake up in the morning. It may take several minutes or more before your C drive increases for you to feel alert. Both C and H drives increase throughout the day. Around 1-2pm, you may feel sleepy since the H drive is high and your C drive takes a ‘small dip’. This is known as “siesta time” which lasts about 20 – 30 minutes, after which the C drive rises, allowing you to feel alert again.

The H drive reaches its highest level at night and the moment your C drive falls, you will feel sleepy and ready for bed. Once you fall asleep, your H drive will slowly fall. Between 2-3am, the H drive is low and can no longer keep you asleep, but you are able to stay asleep due to very low C drive.

Therefore, during the first part of the day you are able to wake up due to a low H drive and during the second half of the day, you are able to stay awake due to high C drive.  Similarly, in the first half of the night, you are able to fall asleep due to a high H drive, and you are able to stay asleep in the final half of the night due to a low C drive.

Knowledge of Circadian and Homeostatic systems can help you feel more alert and sleep better:

1) If you are feeling tired when you wake up, get out of bed right away and turn on lights or get sunlight exposure so that your C drive goes up faster.

2) You may experience tiredness between 1-2 pm (siesta time). A short power nap will help you feel more refreshed. However, even if you don’t nap, you will start to feel better after 20-30 minutes when the C drive goes up.

3) Excessive napping will reduce your H drive, making it difficult to fall asleep at night.

4) If you are working in shifts, you will have low alertness around 3 am. Try to move around at this time so that you do not fall asleep.