The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

Regular poor sleep can raise the risk of serious medical conditions like obesity, heart attack and diabetes - and can even shorten life expectancy. Adequate exposure to natural daylight is known to be crucial for governing the body’s circadian rhythm - the built-in clock which dictates our sleeping and waking patterns.
A key factor to improve one’s sleep and（）