Jet lag is very brief disturbance in sleep as a result of travelling time zones. For example, if you are travelling from India to USA both of, which are in totally different time zones (almost 10.5 hours of difference in time) - In such cases your sleep is scheduled to Indian times, however, the day/night pattern in USA is out of sync to your sleep schedule - i.e., you will feel sleepy and tired from 11:30 AM to 6-7 PM and then start feeling more alert & awake. However, we get re-adapted to the new place within 2 weeks.
There are other disorders similar to jet lag, which can persist for months together - these are called circadian rhythm sleep disorders/irregular sleep-wake cycle disorders. These disorders are difficult to correct, however, can be corrected with effort by the patient and proper direction. First thing is to get proper assessment. I suggest that you see a physician who is trained in sleep disorders. In addition, you could keep a journal and document your sleep (time you go to bed, time you fall asleep, time you wake up) every morning for a period of 2 weeks and then look at the pattern. If your sleep cycle seem to be somewhat shifted either forward or backward (i.e., 6-7PM till 3 AM or 1-2 AM till 10-11 AM) it could be circadian rhythm sleep disorders. Other measure which may be helpful include having set time to wake up and being under sunlight (indirect) for about 30 minutes to 1 hour after awakening. I strongly suggest that you see a trained physician who can diagnose and treat.