From the article: “Chawla and his team analyzed data collected via Fitbit from 557 college students over the course of four years. They recorded 255,736 sleep sessions – measuring bedtimes, sleep and resting heart rate. Significant increases in resting heart rate were observed when individuals went to bed anywhere between one and 30 minutes later than their normal bedtime. Normal bedtime was defined as the one-hour interval surrounding a person’s median bedtime. The later they went to bed, the higher the increase in resting heart rate. Rates remained elevated into the following day.” Increased resting heart rate is a risk factor for cardiovascular disease.