Introduction: Sleepiness is responsible for a considerable proportion of traffi c accidents. It is thus an important traffi c safety issue to fi nd a robust, objective and practical way to estimate the amount of time a person has been awake. To attempt to meet this goal, we investigated the relationship between sleepiness and posture control. Methods: Subjects were kept awake for 36 hours and posturographic data during quiet standing were collected every two hours by means of a force platform. The standing surface (rigid surface or foam surface) and visual (eyes open or eyes closed) conditions were manipulated. Results: In the more challenging conditions (with foam surface and/or eyes closed), the body sway variables derived from the center of the pressure measurement increased signifi cantly when time since awakening became greater than 21 h in almost all subjects. Conclusion: Based on this result, we propose a practical protocol that could robustly assess whether time since awakening was greater than 21 h.