Syncing to our own circadian rhythm for our health's sake

Our circadian rhythm is the 24-hour internal clock in our brain that regulates cycles of wakefulness and sleepiness by responding to light changes in our surroundings.

But how and where exactly in the body does this operate?

The suprachiasmatic nucleus (SCN) is located in the hypothalamus of the brain, above the optic chiasma and serves as the master regulator of our circadian rhythm.

The SCN consist of neuron clusters which have electrical potential frequency that fluctuates spontaneously within an approximate 24-hour period, peaking during the daytime.

We even have a “new light receptor” in our irises, called our melanopsin receptor. It senses light so that we stay awake and it also indirectly connects to the part of the brain that produces melatonin. So, first thing in the morning, our melanopsin receptor cells sense light and tell the SCN that it’s morning and this is time to sync up.

When our body gets the “new light” signal, we start to change our gene expression, with the biological changes syncing up to this circadian rhythm. Hundreds of genes are turned on in the SCN and many neuropeptides are secreted.

Interestingly, the liver clock, the gut clock, heart clock and other organ clocks all time themselves according to our circadian rhythm. These clocks help to time different processes. Eg Bile acid production, hormones synthesis, lipid (fat) metabolism, leptin release, nucleotides to be made, protein degradation, glucose production are all timed to our organ clock’s individual circadian rhythms!

Interesting fact about the timing of our food consumption:

Did you know our liver has 3000-5000 genes that are switched on and off according not to our light-dark cycle cues but according to when exactly our food is consumed? These genes are expressed and specifically timed to when we put food in our belly! Essentially, the time when we eat tells the liver clock genes when to turn on and off. Every organ actually follows when we eat. So, BOTH the first sight of bright light and the first sight of food determine how our body clock works.

Melatonin

Time-of-day information is also transmitted to the pineal gland from the SCN to regulate production and secretion of the indole amine, melatonin.

So what exactly is melatonin?

Melatonin is a hormone synthesized by the pineal gland in our brain and is released in response to darkness. It is critical in regulating our light-dark circadian cycle down to a cellular level, regulating over 5000 genes!

Melatonin is derived from serotonin (our ‘happy’ neurotransmitter/hormone) via two enzymatic steps, and then secreted from the pineal gland at night. The high concentration in our cells’ mitochondria make it arguably our strongest and most powerful antioxidant!

Secretion of melatonin is rigorously controlled by light exposure. Peripheral cells exposed to melatonin respond in night-mode, whereas cells not exposed to melatonin respond in day-mode. Less than 3 lux (light measurement) of light exposure at night is effective in suppressing the onset of melatonin secretion and shortens melatonin secretion duration in humans.

What does <3 lux of light look light?

1 lux is also measured as the equivalent to the illumination of a one metre square surface that is one metre away from a single candle.

Full moon light = 1 lux

Ipad screen = around 100 lux

Sunset and sunrise = 400 lux

As we prepare for sleep, our melatonin level begins to rise. This usually occurs around 2-3 hours before our habitual sleep time.

Melatonin at sleep time also engages with its receptor in the pancreas, which tells our pancreas it’s time to sleep and it doesn’t need to release insulin. As we become less insulin sensitive, it is best that we are not consuming any food at this time, meaning for our blood glucose to be best managed by our insulin secretion it is ideal for us to be finished eating earlier rather than later and for us not to be eating in the dark!

Blood sugar homeostasis is under circadian control at the level of both peripheral tissues and the SCN. Disruption of either the circadian clock or metabolism can lead to derangement of the other, predisposing us to metabolic disorders such as type 2 diabetes and obesity.

Neural (brain) systems such as the limbic brain regions, monoamine neurotransmitters, and the hypothalamic–pituitary–adrenal axis (HPA axis), are all under circadian regulation as well!

Because circadian rhythms do not instantaneously reset, they may remain more closely entrained to the original time zone than the current time zone for several days, with a lag in synchronizing these internal rhythms to the current light and non-light (social interactions, timing of meals etc) cues resulting in disturbed sleep, disrupted hormonal profiles, daytime fatigue, digestive and gut issues, and changes in mood.