Smart phones can get you to the bus station, track flight delays, and monitor your sleep. Now, they may help you adjust to new time zones, too. Released yesterday, a new app called Entrain aims to help travelers restore whacked-out rhythms after long trips. The researchers who developed it got the idea while studying the mathematics of circadian rhythms. Sometimes called your “internal clock,” circadian rhythms tell your body when to eat, sleep, and perform a wealth of other physical functions. You experience jet lag when these internal cycles get out of sync with the time of day (or night) outside, leaving you wide-awake and ready for lunch at 3 a.m. This feeling usually fades as your body adjusts to the new time zone and resets its internal clock accordingly. But the team of mathematicians behind Entrain wondered if they could help you adjust faster by calculating the precise cycle of light and dark that would best ease your body into its new environment. Their number crunching resulted in specialized adjustment schedules for more than 1000 travel itineraries, they report today in PLOS Computational Biology. If you’re traveling from Detroit to London, for example, Entrain says you’ll need to turn off the lights at about 9 p.m. and rise at 6:20 a.m. the next day in order to get used to your new time zone as quickly as possible. If the app prescribes bright outdoor light when you’d rather sleep in, a therapeutic sunlamp in your hotel room might do the trick, the researchers say. If the prescribed schedule doesn’t help your jet lag (or even if it does), you can let them know by logging your travel schedule and jet lag experiences on the app. That data will help the team determine if its theoretical models work in real life.