When someone is awoken by being too warm, the body thinks it’s time to get up for the day because a body associates heat with activity. When someone is awoken by being too cold, the body reacts so that it can get moving and warm up. Our bodies want to make sure that we aren’t freezing to death, after all. But temperature shifts don’t just wake you up—they make it difficult for you to slip into the stages of deep sleep, especially REM. This means you could be losing valuable rest that you need to remain alert the next day and healthy in the long run!