Little was known about sleep until modern science revolutionised our understanding. It turns out that what happens while you are unconscious dictates almost every aspect of your waking health, mood and brainpower.While knowing how to get better sleep and why you need it is vital, the nocturnal world holds some interesting secrets. Here we look at eight things you probably did not know about sleep.1. Testy timesIf young men don’t get enough sleep, they have a level of testosterone that is equivalent to that of someone 10 years older, says British neuroscientist and renowned sleep expert Matthew Walker. That is because a big chunk of testosterone release happens while sleeping. Low testosterone levels affect energy levels and mood – and sperm production.Sleep expert Matthew Walker says if young men don’t get enough sleep, they have a level of testosterone that is equivalent to that of someone 10 years older. Photo: utdallas.eduIn women, too little sleep changes the way reproductive hormones are regulated, although the science is more complicated than a simple “less sleep equals less oestrogen” formula. They may experience a breakdown in the delicate monthly timing system that keeps their reproductive health and moods stable. Those who get fewer than six hours of sleep can see up to a 20 per cent reduction in ovulation-inducing hormones.Lack of sleep disrupts our metabolic systems quickly. Getting only five hours of sleep for four nights in a row can negatively affect blood sugar levels, reducing insulin sensitivity to the extent that one could be classified as pre-diabetic, Walker says.The immune system is also affected: restricting a healthy person to just four hours of sleep for a single night results in an alarming 70 per cent drop in the activity of natural killer cells – cytotoxic white blood cells that act as the immune system’s first line of defence.2. Justice for owlsSome sleep scientists feel that modern society discriminates against those whose circadian rhythms – or internal clocks – make them night owls rather than larks who are up at the crack of dawn and go to bed early in the evening.