The circadian rhythm is a natural internal process that regulates the sleep-wake cycle in 24 hours.

The circadian clock is located in the suprachiasmatic nucleus (SCN) located in the hypothalamus. The SCN receives information through specialized ganglion cells in the retina that are directly photosensitive. These cells contain the photopigment melanopsin and their signals follow a pathway called the retinohypothalamic tract.

According to Dr Andrew Huberman, the retinal ganglion cells are brain neurons that respond best to sunlight especially the contrast between the yellow and blue colours of the low solar angle of sunrise in the morning.

Most people tend to wake up within 1 hour of sunrise and adrenaline pulses are released in the body and in the brain and cortisol pulses released by the adrenal gland. A timer is set for Melatonin secretion by the pineal gland 12 to 14 hours after. The pineal gland is a gland of pea size in the 3rd ventricle, it is the only organ to secrete Melatonin. Melatonin helps us to fall asleep but it does not help to stay asleep.

If we wake up late, we still need to go outside for sunlight exposure about 2 to 10 min. Once the sunlight is overhead, it is late so we miss the early shift of cortisol. The early increase of cortisol in the day is beneficial to reduce anxiety and depression.

Blind people, as long as the eyes are not removed, still have this ability as the melanopsin cells can send signals to set the clock.

By viewing the sunlight within 1 hour of the sunset, the melanopsin cells in the retina are stimulated and this protects us from the negative effects of light exposure at night.

Exposure to light late in the day has bad effects on the circadian rhythm and sleep. The longer we stay awake, the more sensitive our retina is to light. A small amount of light on the screen may disrupt the rhythm and make us want to stay up late. For a good sleep, we want less light from 8pm and no light from 11pm to 4 am as it suppresses the release of dopamine and inhibits learning. It stimulates the habenula or disappointment nucleus that can trigger depression.

Red light does not trigger this effect.

The location of light is also important as the melanopsin cells are mostly located in the lower half of the retina to view the sunlight in the upper space. In the evening, it is better to have lower light in the bedroom as overhead fluorescent light is worse. Candle light or fire place light is fine. If we wake up in middle of the night, it is better to use a dimmer light.

The circadian clock can be manipulated with light exposure. In a study in teenagers who sleep late, if the light was shown on them before waking up even with close eyes, it makes them reset the clock and want to go to sleep earlier.

Therefore, if we are exposed to light late in the day, it will delay the clock and it is hard to wake up earlier. If we wake up early and are exposed to sunlight, it will advance the clock. Generally, it takes a few days to reset the rhythm.

Sometimes there is a peak of alertness about 1 hour before bedtime probably a signal to gather things before sleep or to organise things for the next day.

Caffeine acts as adenosine antagonist by blocking the sleepiness receptors. Sleepiness is driven by adenosine naturally. When adenosine binds to its receptors, the neural activity slows down and we feel sleepy. Adenosine level is creeping up slowly the longer we are awake. It facilitates sleep and dilates the blood vessels to ensure good oxygenation during sleep.



Compounds that can help with sleep according to Dr Huberman:

- Magnesium L threonate 300-400mg 30- 60min before sleep.

- L Theanine100-200mg

- Apigenin (Chamomile) 50mg



Reference:

Huberman Lab Podcast

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