Circadian rhythm disorders are the result of the mismatch between the external environment and our internal biological clock. This clock is the pacemaker system that controls many different bodily functions during a 24-hour period and regulates when we sleep and when we wake up. Jet lag is one of the most common circadian rhythm disorders. It occurs after a plane flight that crosses several time zones so that your body clock thinks it is in your departure time zone, but you are actually in another time zone. Symptoms of jet lag may include excessive sleepiness, lethargy, insomnia, headache, loss of appetite, gastrointestinal problems, and irritability or mild depression.