Flying across time zones changes the principal time cue -- light -- for setting and re-setting our 24-hour, natural day-night cycle, or circadian rhythm. Our internal clock becomes out of sync or mismatched with our current day-night cycle. Our circadian rhythm greatly influences when we sleep, and the quantity and the quality of our sleep. It may also be altered by the timing of various factors, including naps, bedtime, and exercise.
In general, "losing" time is more difficult to adjust to than "gaining" time. Traveling east we lose time; west we gain. An "earlier" bedtime may cause difficulty falling asleep and increased wakefulness during the early part of the night. Going west, we fall asleep easily but may have a difficult time waking.