The temperature of a healthy body is not really stable during the day and tends to go up and down a little. The same is true while you sleep, although the mean tends to be some 1 to 2 degrees lower at night than during the day.
Body temperature starts to fall naturally as bedtime approaches, paving the way for a good night’s sleep. Your body also tends to lose heat when you are quiet and at rest, which helps you fall and stay asleep: we sleep better when we’re cooler. And by the way: this is one of the reasons why experts say you should not exercise close to bedtime: exercise will heat you up and contrast the natural cooling down before sleep.
Your temperature then starts to rise again naturally towards morning, preparing your body for wakefulness. Some people can be particularly sensitive to this change, leading them to wake up in the early hours of the morning feeling too hot.
Some types of mattress can make body temperature control more difficult than others. Foam mattresses in particular trap warm air between your body and the mattress itself. The warmth is what makes the foam mold to the shape of your body and makes the mattress more supportive. But this same warmth is reflected back to your body and may make you feel overheated.
Mattresses with natural materials on the contrary allow proper perspiration and temperature control. Materials like wool, linen and cotton help keep your body fresh at the naturally regulated temperature. So you are less likely to be woken up by a hot sweat in the very early hours of the morning.