What happens to your body when you don’t get enough sleep?

Too little sleep can affect multiple body systems, especially when it is repeated:

• Hormones and metabolism: Five nights of four to five hours of sleep were associated with lower sex hormones; four nights of about five hours impaired blood-sugar control enough to reach a prediabetic range. 1
• Immune defenses: One night of four hours of sleep was linked to a 70% drop in natural-killer-cell activity. Short sleep was also associated with a weaker response to flu vaccination and greater likelihood of catching a common respiratory infection. 2
• Heart and cellular health: Sleep loss was associated with cardiovascular risks, and one week of six-hour nights altered the activity of 711 genes, including genes connected to immunity, inflammation, tumors, and cardiovascular disease. 3
• Appetite and weight: Sleep restriction can increase desire for less healthy foods and overeating; one review cited roughly 250–400 extra calories a day, and a two-week study found weight gain with about five hours of sleep nightly. 4 5
• Mood and emotional regulation: Insufficient sleep can make emotions harder to manage and mood more volatile. 6
• Muscle and exercise: When sleep-deprived people diet, a greater share of their weight loss may come from lean muscle rather than fat; undersleeping can also reduce motivation to exercise and raise injury risk. 7 8

A single poor night can produce measurable changes, but the most concerning risks described here involve sustained or repeated sleep loss. 9