What's the best temperature for sleeping?

Essentially, a slightly cool room and a lower core temperature seem to be optimal for sleeping. The best temperture for most adults trying to get to sleep is in a slightly cool room at around 64° F or 17.7° C with good ventilation.

In general, room temperatures in the range of 60 to 68 degrees Fahrenheit have been found to be the optimal temperture for sleeping. While that may seem quite cool to some, this cool temperture helps facilitate the decrease in core body temperature that in turn initiates sleepiness. For some people, temperatures that fall too far below or above this range may lead to restlessness.

Also if you sleep too hot, you will often sweat, which can lead to body acne or backacne if you sleep on your back mostly.  That's why it's a good idea to sleep with the minimum amount of apparrel on to keep you sleeping well at night.  For instance, don't wear socks to bed and if you wear a t-shirt or a top to bed, make sure it's a clean one every night.  Also it's a good idea to change your bed sheets once a week.