#free-running.

What Is Non-24-Hour Sleep-Wake Disorder?

A plain-language guide to Non-24-Hour Sleep-Wake Disorder (N24): what it is, what "free-running" means, why sighted people get it too, how it's diagnosed and managed, and why several of its stranger features — marathon wake periods, sleep debt that won't clear, a sleep window that slams shut — are completely normal for the disorder.