A decrease in core body temperature (caused by a cooler room) is a signal to sleep. During a regular day, your body temperature rises and falls naturally. As it starts to fall, you become drowsy. Your body temperature gets to its lowest level at 5 a.m., and then climbs 2 or 3 degrees as the morning wears on.
If your bedroom is too warm, you’re likely to wake in the night. When your core temperature (the temperature of your brain and abdominal cavity, as opposed to your skin temp) gets too high, you start to sweat, which cools down the body but keeps you from a restful sleep.