In 2023, Daniel P. Windred and a team studied sleep patterns in thousands of adults across multiple cohorts, finding that sleeping in on weekends, called social jetlag, disrupts your body’s internal clock. Each hour of jetlag raises heart disease risk by 11% and worsens mood, obesity, and unhealthy habits like smoking or poor diet. A 2019 study by C.M. Depner showed that catching up on sleep after five short nights still caused 10-15% worse insulin sensitivity and higher calorie intake, leading to weight gain risks.

In 2015, J. R. Dalenberg and a team from Denmark conducted two field experiments with 30 students in dorm rooms. They tested ventilation by opening windows (low CO2: 660 ppm) or using a fan (low CO2: 835 ppm) vs. no ventilation (high CO2: 2,585 ppm or 2,395 ppm) for 1 week each. Sleep was tracked with wrist actigraphs, and next-day alertness via questionnaires and cognitive tests.