a. Jet lag
Jet lag occurs when the body's biological clock is out of sync with local time. When traveling to a new time zone, our bodies are slow to adjust, and remain on their original biological schedule for several days. The result is that we feel excessively sleepy during the day or stay wide awake at night.
A psychiatric or medical disorder can disrupt the rhythm and lead to disturbed sleep. c. Shift working
A shift worker is anyone who follows a work schedule that is outside of the typical "9 to 5" business day. As a result of untimely work schedules the shift workers have disturbed sleep rhythms and face frequent sleep disturbance and associated excessive sleepiness.