Jetlag is definitely the downside to travel. It is the feeling of being tired and disoriented after a long flight.
A person usually becomes jetlagged from flights that cross multiple time zones. Long flights cause your sleep pattern to change and other discomforts such as sore muscles and exhaustion.
These effects may be felt for a few days until your body adjusts to the time change. For example, South Africa is 6 hours ahead of New York time in our summer (7 hours during our winter). So, when it is 9:00 am in Johannesburg, it is either 2:00 pm or 3:00 pm in New York depending on the time of the year.
Once a traveler adjust to the time change they usually start to feel much better!