Why do we get jet lag?

The body runs on a circadian rhythm of roughly 24 hours, controlled by the suprachiasmatic nucleus in the brain and set mainly by light. When you fly across several time zones, your body clock still follows your home schedule, so you feel sleepy, hungry or alert at the wrong times. Symptoms include poor sleep, fatigue, headaches, low concentration and digestive upset. The body typically adjusts at about one time zone per day, so crossing six zones may take close to a week to recover fully. Flying east is usually harder than flying west, because it is easier for the body to lengthen its day than shorten it.