Trait overview: melatonin, sleep and glucose

Tuesday, June 23, 2020. Author FitnessGenes

Man eating late night meal

How do we know when it’s time to go to sleep? 

The answer to this depends on multiple factors, including our work routine, our stress levels and whether we’re naturally more of a ‘morning lark’ or a ‘night owl’.

Nevertheless, and even with the advent of artificial light, most of us are typically awake during the day, when it is light outside, and asleep at night, when it’s dark. This coupling of our sleep pattern to levels of light is regulated by an important hormone: melatonin.

Recent research suggests that melatonin also coordinates the secretion of insulin with our sleep-wake cycle. Our gene variants influence the relationship between melatonin, sleep and insulin release, with some variants linked to poorer insulin function and higher blood sugar levels.


What is melatonin?

Often dubbed ‘the sleep hormone’, melatonin is a chemical messenger that is produced at night and makes us sleepy.

It also coordinates the control of other physiological parameters with our sleep cycle, such as our blood pressure, metabolism and body temperature.


How is melatonin produced?

Melatonin is produced by a small gland located deep in the centre of the brain called the pineal gland.

During daylight hours, the pineal gland is inactive and produces very little melatonin. During the night, however, when darkness falls, the pineal gland becomes activated and increases its production of melatonin.

For this reason, melatonin is sometimes (jokingly) referred to as the “hormone of darkness” or the “Dracula hormone”.


How is melatonin production coupled to daylight and darkness?

There is a nifty mechanism that ties melatonin production to light levels. When it’s dark, less light falls on our retinas – the light sensitive layer at the backs of our eyes. This drop in light levels is then signalled to another part of the brain called the suprachiasmatic nucleus (SCN).

The SCN, which is found in a brain structure called the hypothalamus, is sometimes known as the “master clock”. It is our main “internal body clock” that regulates circadian rhythms: hormonal, physical and behavioural processes which follow a 24 hour-cycle, such as our sleep-wake cycle or body temperature.

During darkness, the SCN stimulates the pineal gland to produce and secrete melatonin. This, in turn, enters the bloodstream and has various effects in the body, including making us feel sleepy, reducing our core body temperature and inhibiting the release of insulin.

By contrast, bright light (both sunlight and artificial light) causes the SCN to suppress the production of melatonin by the pineal gland.

Levels of melatonin therefore stay high in the bloodstream during the night-time, until the re-emergence of daylight in the morning begins to inhibit melatonin secretion.


How does melatonin affect insulin secretion?

Melatonin inhibits the secretion of insulin.

Insulin is a hormone produced by the beta cells of your pancreas. These beta cells produce (or ‘express’) a receptor for melatonin, the MT1B melatonin receptor, which is coded for by your MTNR1B gene.

When levels of melatonin in the bloodstream are high during the night, melatonin binds to the MT1 receptor and suppresses the secretion of insulin by pancreatic beta cells. As a result, your circulating levels of insulin drop overnight. 


Why does melatonin affect insulin secretion?

A major action of insulin is to facilitate the movement of glucose from the bloodstream into tissues. Insulin, therefore, causes levels of glucose circulating in your bloodstream to fall. 

A steady overnight supply of glucose in the blood, however, is required by your brain, which is very active during sleep. Unlike other tissues e.g. skeletal muscle, brain tissue does not require insulin to take up glucose from the bloodstream for fuel.

By inhibiting insulin secretion during night time, melatonin helps to keep blood glucose levels adequately high, ensuring your brain is well fuelled during the night.    


How do variants of your MTNR1B gene affect insulin secretion?

Your MTNR1B gene codes for your MT1B melatonin receptor, which is found on the surface of beta cells in your pancreas. When melatonin binds to this receptor, it inhibits the release of insulin into the bloodstream.

Different variants (or alleles) of your MTNR1B gene affect how many MT1B melatonin receptors your beta cells produce. One allele of the MTNR1B gene, the G allele, increases the amount of MT1B receptors expressed on the surface of your pancreatic beta cells.

When melatonin binds to these greater number of receptors, it more strongly inhibits the secretion of insulin. Resulting in lower circulating levels of insulin during night time.

Issues then arise when people with the G allele eat a large, carbohydrate-rich meal late in the evening when their insulin secretion is suppressed by melatonin. In this scenario, blood sugar (glucose) levels may become excessively high, as lower levels of insulin struggle to transport glucose from the bloodstream into tissues.

If high blood glucose levels persist over time, their risk of metabolic disease such as Type 2 diabetes and metabolic syndrome increases.


The melatonin, sleep and glucose trait

Depending on the MTNR1B gene variants that you carry, the FitnessGenes melatonin, sleep and glucose trait will categorise you into one of three possible trait bands:

  1. High risk of insulin-sleep cycle dysfunction.
  2. Moderate risk of insulin-sleep cycle dysfunction.
  3. Low risk of insulin-sleep cycle dysfunction.

For those with the highest risk of insulin-sleep cycle dysfunction and consequently metabolic disease, critical actions to take may include:

  • Scheduling your evening meal at least 4 hours before going to sleep.
  • Avoiding large amounts of fat and carbohydrates in your evening meal, as this can slow the absorption of carbohydrates.
  • Optimise your zinc levels by considering your zinc trait, as the production of insulin requires zinc.


Discover your personal trait

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