Not getting enough sleep can make you more than tired: It can put you at risk for type 2 diabetes and memory loss. University of Chicago researchers found that when study patients were only allowed to sleep four hours for six days in a row, their bodies had a hard time metabolizing carbohydrates, causing blood-sugar levels to rise. The sleep debt also caused an overproduction of the stress hormone cortisol, which helps regulate blood-sugar levels. What’s more, high cortisol levels have been linked to memory loss. So how much sleep is enough? Most people need about seven to nine hours a day.