In spite of the mainstream attitude “I will sleep when I am dead”, sleep is not optional. It is essential for optimal health and a key requirement for mental and physical peak performance. Sleep impacts all body functions from hormone production to immune system to circadian rhythm (described later). This guide explains why sleep matters and how to get perfect sleep.
The Cost of Sleep Deprivation
Sleep deprivation kills your brain cells first – then you
Lack of sleep is related to mental diseases such as depression and Alzheimer’s. Sleep deprivation is a proven way to damage your brain by suppressing the creation of new brain cells (neurogenesis) and can kill rats within 3 weeks in the lab. Diseases such as fatal familial insomnia and Morvan’s syndrome suggest that sleep deprivation ultimately can kill humans too. (Although there are no human studies yet, probably for ethical reasons). And indeed, too little sleep is associated with increased mortality [2,3,4].
Lack of sleep triggers cravings, promotes obesity and type-2 diabetes
There is a strong link between too little sleep and obesity too. While people with sleep disorders have a higher risk for developing obesity  also healthy men and women exhibit an inverse relationship between sleep duration and body weight [2,3,4,5]. This is valid even in children. As little as five days of insufficient sleep are enough to increase your body weight. A causal mechanism is that sleep restriction alters the circulating levels of the hormones leptin and ghrelin  that in turn increase appetite, caloric intake, reduce energy expenditure and impair glycemic control. That is why sleep deprived humans are more susceptible to junk food and experience food cravings and blood sugar swings. Sleep loss increases the risk for type-2 diabetes [2,3,4,5], by decreasing your insulin sensitivity and changing your gut microbiome.
Increased risk for heart disease and cancer
Sleep deprivation promotes inflammation by increasing C-reactive protein (CRP) (a common marker for inflammation). If you don’t sleep enough you increase your risk for cardiovascular diseases  and hypertension. Irregular sleep practices and low melatonin at night are contributing to heart disease and cancer [2,3,4].
Poor sleep kills your sex drive and makes you weak
Sleep deprived humans have a weaker immune system and are up to 550% more likely to develop a cold. Poor sleep is related to increased secretion of the stress hormone cortisol. At the same time it lowers the levels of the sex hormone testosterone and 1 in 3 women find themselves too sleepy for sex. Lack of sleep accelerates aging and also impairs your empathy and ability for social interaction.
In short: Lack of sleep makes you stupid, weak and fat
Sleep deprivation reduces productivity and causes accidents
Lack of sleep impairs cognitive function, attention span and motor skills  and contributes to errors at work, injury and accidents, including 20% of all motor vehicle crashes in the USA [2,3]. As little as one night without sufficient sleep is enough to cause these effects. Fun fact: 24 hours without sleep is the cognitive equivalent of being drunk. These effects are cumulative and your performance declines steadily with each additional night of too little sleep. 14 days of 6 hours sleep per night cause the same drop in performance as 48 hours without sleep. Interestingly, sleep-deprived people are poor judges of their decreased performance and overestimate their productivity.
Although you might feel fully awake and content with your sleep-deprived performance it is not optimal.
The Purpose of Sleep
Sleep clears toxins from your body
Similar to the bodies lymphatic system your brain has its own system to remove toxins. During sleep, the so-called glymphatic system uses spinal fluid to clear neurotoxins from your brain [2,3] that accumulate in the awake central nervous system (here is a fascinating TED talk explaining this process). The removed toxins include amyloid beta, the main component of the amyloid plaques found in the brains of Alzheimer patients. The restorative function of sleep may be a consequence of this enhanced removal of waste products.
Sleep repairs and regenerates your cells
Sleep at night is a truly anabolic state as levels of growth hormone and testosterone are increased. Good sleep also promotes healthy skin and controls optimal insulin secretion. It supports healthy cell division which helps prevent cancer. Not only your muscles get repair and recover but also your adrenal glands, your immune and your detoxification systems get restored.
Sleep enhances your physical and mental performance
Adequate sleep improves athletic performance. More sleep is an effective way to lose fat: People who slept 1 hour more each night instead of watching TV lost 6.5kg in 1 year.
Furthermore, good sleep not only enhances your ability to learn new motor skills but also enables you to come up with novel solutions to complex problems . Most importantly longer sleep directly increases your IQ.
Optimal sleep contributes to better mood & health, improves your capacity to handle stress and learn new things.
You can read Part II here.