Lack of sleep contributes to increased levels of anxiety and stress. A moderate amount of stress or good stress—the kind that makes you feel excited—can help you deal with difficulties or unexpected events. But when stress is excessive and continuous, it can weaken your immune system. Sleep has a lot to do with certain immune functions and a healthy immune system. Research shows that only three nights without sleep affects your T cells. T cells help the body distinguish between friend and foe. They also work with natural killer cells to attack harmful invaders. When you sleep, your body behaves differently than when you are awake. During this time, your body regenerates and repairs cells. That’s why you feel exhausted when you don’t get enough sleep, which may lead to health problems.