The human body has evolved to survive and flourish in its environment over the last two million years from the time when our primate ancestors left the trees and migrated to the planes of Africa.

The truth is that our genes have adapted a little since this time, but evolution is slow and our genes are still very much the same as they were 10,000 years ago.

10,000 years ago was just 1,500 years after the last Ice Age and to our knowledge things were very much simpler than life is today.

For most humans, the day would involve waking up, hunting for food, preparing food, building shelter, making clothes, looking after younger family members, learning from our elders and, when safe enough, to play.

It was this kind of environment in which our genes developed. The earliest evidence of our Homo genus ancestors making fires dates to around 1.6 million years ago. But today, we are bombarded by light sources of all types. So what has light got to do with your hormones? Don’t worry, I’m getting there.

Today, we use artificial light for street lights, car headlights, indoor lighting, TVs, computers, tablets, mobile phones, computer consoles and so on and so on.

10,000 years ago, it was likely that our ancestors would have used fire, but not to the extent of our use of artificial light today. It is also likely that our ancestors would have gone to sleep soon after sun down and woke at sunrise. There were no late night TV shows or alarm clocks in those days!

Whilst new born babies sleep for 16-18 hours a day, at around 3 months of age, humans begin to recognise day as day and night as night – the circadian rhythm!

During a natural 24-hour circadian rhythm there are a number of hormonal changes that occur.

At sunrise, the light receptors on our skin and in our eyes are stimulated. The receptors are part of the central nervous system and they signal to the brain that it is time to get up and start the day. In addition, the signal to the brain also stimulates the release of cortisol (a stress hormone) which helps makes us more alert and ready to hunt for the day’s food.

A normal circadian rhythm

As you can see from the diagram above, cortisol (the black line) peaks and then begins to drop dramatically around 8.30am to a low level by lunchtime. Could it be that when we were evolving it was just too hot to hunt in the midday sun?

And by sundown, cortisol levels would have plummeted to their lowest levels. At the same time (sundown), the body begins to release melatonin (melatonin release is inhibited by light) and this growth hormone triggers the release of other growth hormones, such as testosterone, estrogen, and human growth hormone.

Growth hormones are essential for the repair and regrowth of tissues and cells that were damaged or died that day. A lack of growth and repair and the body will function less and less effectively over time.

It’s fair to assume that our ancestors slept longer in the winter and shorter in the summer, but could have slept anywhere from 8-12 hours per day in total darkness (no street lights in those days).

In today’s society, things are very different. Most of us today go to bed long after sundown, often as late as the early hours of the next day. The diagram above indicates that growth hormone levels (the white line) peak between 10pm-2am, which is the time when our physical body repairs itself, but only if growth hormone levels are adequate.

Our psychological repair tends to occur around 2am-6am in the morning.

As you can see from the diagram below, we start the day with high levels of cortisol (if we are lucky), but due the stresses of modern day life these often stay elevated. This could be due to work stress, financial stress etc.

A stressed circadian rhythm

Later in the day, we don’t take the time unwind after sundown and use artificial light well into the night. We then go to bed late often due to watching late night TV or working long hours or even shift work.

This eats greatly into our window of opportunity to stimulate the release of growth hormones. This means we don’t get our full quota of physical repair. Think of your body like a building. If you had a building and you didn’t keep on top of its maintenance, the building would start to look shabby and eventually, start to fall down.

It is the same with our bodies. If we don’t get our full quota of physical recovery each night, our bodies over time will start to breakdown. A lack of sleep can lead to:

[dt_cell width=”1/2″]

  • A loss of muscle mass
  • Increased body fat
  • Lethargy
  • Incomplete recovery from exercise
  • Overtraining
  • Loss of performance
  • Increased likelihood of infections

[/dt_cell][dt_cell width=”1/2″]

  • Loss of libido / sexual performance
  • Infertility
  • Joint aches
  • Gut problems
  • Anxiety
  • Cognitive problems
  • And many more symptoms…


So the point is that not only do you need to get enough sleep, but you need to get it at the right time. You can’t go to bed late and expect to catch up by waking up later. You just can’t cheat nature and your circadian rhythms!

I recommend that you begin to unwind in the early evening, avoid artificial lights within 1-2 hours of bedtime and be in bed by 10-10.30pm at the latest each day to maximise the window of opportunity for growth and repair.

Leigh Brandon

Leigh Brandon has been working in the field of health and fitness since 1996. His mission is to help as many people as possible to reach their health and performance potential, ‘so they can live more productive, fulfilling and happier lives’.

Latest posts by Leigh Brandon (see all)