1. But the German study, which used detailed sleep recordings of 188 patients who lived in urban settings and suffered from disturbed sleeping patterns, found that even when exposed primarily to artificial lights, the participants experienced seasonal variations in REM sleep, which is directly linked to our circadian rhythm. In fact, the participants slept an hour longer in December than in June. Their rapid eye movement (REM) sleep, which is the most active stage of sleep when we dream and our heart rate increases, was 30 minutes longer in the winter than during the summer. REM sleep is regulated by the circadian clock, so the fact that it goes in parallel with seasonality makes sense, says Kunz.The German scientists say children may benefit from an earlier bed time in the winter (Credit: Getty Images)But Kunz's team was surprised to find that there were also seasonal changes when it came to slow wave sleep (SWS), also known as deep sleep. We found specific changes in REM sleep and deep sleep, the two major stages in sleep, over the year. This was completely new, says Kunz.