Biological Rhythms All plants and animals have endogeous cycles that, in response to an outside rhythm (zeitgeber), will cue various actions. For example, bears know they must hibernate, poinsettias know they must flower and humans know they need sleep. In correspondence with a particular season, length of day, and light intensity respectively, these events will occur. In the example with the poinsettia, the vegetative/flower cycle is synchronized with a day length cycle that has …showed first 75 words of 1036 total…

…showed last 75 words of 1036 total…particular season. The occurrence of SAD during the summer months is a very rare condition. Typically, winter is the season when the majority of sufferers feel the onset of SAD. The days get shorter and the nights get longer as the transition from autumn to winter takes place. As said earlier, melatonin production (which occurs during the nighttime) causes you to feel sleepy and less alert and can onset the feeling of depression and irritability.