Jetlag occurs when we fast-forward or drag back in time our circadian rhythm. It’s a disruption of our circadian rhythm as a result of our crossing time zones. For that reason,  we struggle to function in the new time zone because there is a mismatch between our internal circadian clock and external time (new time zone) . So, you can be sleepy in the day in the new location because you are biologically supposed to be sleeping. Or, you can’t sleep at night because it’s daytime in your biological circadian rhythm time zone.

Arendt, J., & Marks, V. (1982). Physiological changes underlying jet lag. British Medical Journal (Clinical Research Ed.), 284(6310), 144–146.