When our internal clock breaks its stride, it makes us more than just tired: Researchers have found that a daily rhythm out of step has negative consequences. Whether it is time to go to sleep or to be active, our body knows this without an alarm clock. An internal clock controls our sleeping and waking phases and is oriented primarily to the natural rhythm of day and night. If this rhythm is disrupted, for example by jet lag or shift work, this also has physical consequences: A disturbance of the internal clock promotes obesity, for instance, and can even accelerate mental decline.
Metabolic Balance Blog 