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.