You might already be trying to keep it cool at night (if you have AC) during the summer as best you can & if you are it might not be great for your PG&E bill, but it looks like it's better for your health. Per the Harvard Medical School, sleeping in a cold room will help you fall asleep faster. Your body is already trying to cool down right before sleep so this helps to expedite that process, therefore putting you to sleep quicker.
What counts as a cold room? The U.S. National Institute of Health would consider that 66 degrees & they say that sleeping in a room at that temperature will increase your metabolism & reduces the risk of metabolic illnesses.
Oh, and sleeping in a cold room can help reduce insomnia. Insomniacs have a higher body temperature when they're sleeping and the University of Southern Australia deduced that a lower room temperature can help lead to a small temperature change in an insomniac's body, which is crucial to helping them drift off to sleep.
Sleep Advisor says that all of the above leads to reduced stress throughout the day - and of course it does, you're more well-rested.