After a long flight that crosses many time zones, your body still thinks it is the old clock. You feel tired at the wrong hour, your head is foggy, and sleep will not land where you want it. That mess is jet lag; a person who has it is jet-lagged. It is ordinary English, not a doctor's word. Flying east, toward an earlier morning, is often harder than flying west. The feeling can last a day or two, sometimes more. You can also say you have jet lag or you are suffering from jet lag. Though actually, a short hop that stays in the same time zone does not really give you this — tired from a plane is not the same thing as jet-lagged.