The onset of drowsiness is the moment when staying awake requires a conscious effort, usually an effort to keep the eyes open. This moment is the last step in falling asleep, not the first. In any hazardous or potentially hazardous situation such as driving, the onset of drowsiness should be regarded as a red alert- a powerful signal to get out of harm’s way instantly. Strong external and internal stimulation during wakefulness can mask a large sleep debt and a strong underlying tendency to fall asleep. If drowsiness occurs soon after a transition from being active and moving about to a sedentary situation such as driving, your sleep debt is very likely huge and dangerous.