Sleepiness mainly comes from a deficit in sleep quantity or quality. From a homeostatic perspective, obtaining enough sleep of good quality is the most determining factor. The circadian perspective explains why your body clock contradicts your wish to sleep at appropriate times once at destination. It may make it difficult to fall asleep (Group A), to wake up (Group C), or both (Group B). Here is what you can do about it, using sleeping pills if needed (benzodiazepines and others).