How do our bodies know when to go to sleep?

A piece I wrote for the British Council Blog prior to FameLab International.

How do our bodies know when it’s night?

In our eyes, there are photoreceptors: cells which can determine the difference between light and darkness. But how do the kidneys, liver or stomach know when it’s dark or light?

During periods of darkness, the photoreceptors in our eyes send a signal to a part of our brain called the pineal gland, which starts the production of melatonin. Melatonin is known as the ‘hormone of the night’. It’s a chemical expression of darkness. It acts as an endocrine hormone, meaning that it’s released into the blood. While circulating in our body, it chemically informs all our organs that it’s dark outside and all body parts should be getting ready to sleep.Because melatonin’s activity is a natural way of regulating the ‘body clock’, in some countries this hormone is sold as a supplement for people with insomnia to help their bodies go to sleep.

This same hormone is produced in a range of species: animals, plants, and even microbes. In other mammals, melatonin can play a variety of physiological roles. For instance, initial studies on melatonin using hamsters showed that increased consumption of the hormone shrinks hamsters’ testicles. While this may sound menacing to human males, is it necessarily a cause for concern?

Does melatonin really shrink testicles?

Let us first look at the reasons for these findings. Feeding hamsters increasing amounts of melatonin is a signal for their bodies that periods of darkness (i.e., nights) are getting longer and longer. In nature, progressively longer nights (and therefore shortening days) are a sign of approaching winter. During winter, hamsters, like many other animals, undergo what is called a daily torpor – a period of lowered metabolism and decreased body temperature. This metabolic shut-down happens because in winter, food is scarce, and in order to survive, animals need to focus solely on survival. They will therefore not invest energy in any non-essential body parts, such as testicles. After all, winter is not the best time to think about reproduction or impress the lady hamsters with big gonads! So melatonin does indeed cause hamsters’ testicles to shrink.

But does melatonin work in the same way with humans? Are men who pop melatonin pills likely to experience shrinking testes? The short answer is ‘no’. Humans and hamsters are very different species. Unlike hamsters, we are not seasonal breeders; our physiology is not governed by light to the same extent as theirs. We don’t go into torpor in winter. Men’s testicles remain the same size throughout the year and so do not naturally shrink when days become shorter. In short, melatonin consumption is not going to have an effect on human testes. The lesson here is that we need to be very cautious when interpreting the results of research on animals, as they are not always applicable to humans.

Why is researching melatonin useful?

Apart from assessing the direct benefits of melatonin for humans – melatonin as a sleeping pill, melatonin as an antioxidant, and so on – there are other reasons for studying this chemical.

Because I am a conservation biologist, I find research on animal reproduction fascinating and potentially vital for the captive breeding of endangered species. Our knowledge of how melatonin affects the reproductive cycles plays an important role in the development of assisted reproductive techniques widely used in conservation. Even little things such as lights left on at night in zoo enclosures might have an effect on the reproductive physiology of some animals, by affecting their melatonin levels.

It is simply amazing how such a tiny compound can be so universal, and so powerful.