According to neuroscientist Matthew Walker, not sleeping enough makes you:

… makes you dumber, more forgetful, unable to learn new things, more vulnerable to dementia, more likely to die of a heart attack, less able to fend off sickness with a strong immune system, more likely to get cancer, and it makes our bodies literally hurt more. Lack of sleep distorts your genes, and increases your risk of death generally, he said. It disrupts the creation of sex hormones like estrogen and testosterone, and leads to premature aging. Apparently, men who only sleep five hours a night have markedly smaller testicles than men who sleep more than seven.

“Sleep loss will leak down into every nook and cranny of your physiology,” he said. “Sleep, unfortunately, is not an optional lifestyle luxury. Sleep is a nonnegotiable biological necessity. It is your life support system.”

Read more of the Emily Dreyfuss’ article on Wired