<p>Climate change is one of the most important challenges of our time. The associated rise in temperature is now around 1.5 K in Germany too and heavy rainfall events are becoming more frequent, which in turn increases relative humidity. Due to a&#xa0;lower distal heat dissipation, the decrease of the core body temperature will be lower at higher temperatures in the bedroom, especially when the relative humidity is high. This can prolong sleep latency, results in longer waking times during the night and appears to have an effect on the proportion of deep and rapid eye movement (REM) sleep. The importance of bright, blue light for the circadian melatonin rhythm is well documented. Accordingly, possible changes in sunshine duration and cloud cover must also be considered when analyzing the effects of climate change. In northern German federal states, the degree of cloud cover decreased in spring and summer, but was formally unchanged in winter. Individual regional analyses show that the meteorological definition of seasons—e.g. “winter” includes the months of December to February—does not reflect the decreasing sunshine duration between October and January. Additionally, not only the number of days with 100% cloud cover increased, but also the number of consecutive days without sunshine. This ultimately corresponds to a&#xa0;“biological” darkness with effects on REM sleep, reaction time, and greater social jet lag. As melatonin secretion is primarily controlled by light, these changes can lead to further instability of the circadian rhythm in people with already reduced melatonin secretion—such as those with chronic insomnia—and thus exacerbate and/or maintain the sleep disorder.</p>