Jet lag is the travel hangover that no amount of coffee can cure. You land in a new city, your body thinks it's 3 a.m., and your brain is foggy while everyone else is sipping espresso. The usual advice is to tough it out, but a growing number of travelers are turning to an app that flips the script: instead of focusing on sleep, it focuses on light.

Timeshifter, a jet lag app co-founded by Harvard sleep scientist Steven Lockley, uses personalized plans built around light exposure and light avoidance to help your internal clock catch up faster. The idea is simple. Your circadian rhythm, the internal clock in your brain's suprachiasmatic nucleus, regulates when you feel sleepy, hungry, and alert. When you cross time zones, that clock stays on home time, and it only shifts about an hour per day on its own. That's why a trip from New York to Tokyo can leave you groggy for a week.

Timeshifter's algorithm, the same one used by NASA astronauts, tells you exactly when to seek bright light, when to wear sunglasses, when to nap, and when to skip caffeine. The plan starts the day before your flight and continues through your trip. According to the company, travelers who follow the advice are 14.1 times less likely to report severe jet lag, and 96.4% of users cross time zones without feeling jet lagged at all.

Former NASA astronaut Mike Foreman swears by it. “As an astronaut I did a great deal of traveling internationally,” he says. “NASA had access to doctors and scientists who developed algorithms that gave us written schedules to follow. These are the same algorithms used by Timeshifter.” Foreman, now retired, says he uses the app religiously when he travels.

The app is fully personalized. You enter your flight details, including stopovers, plus your age, sleep habits, caffeine and melatonin use. Then it generates a multi-day schedule. The goal is to realign your circadian rhythm with local time as quickly as possible, potentially gaining up to four hours of adaptation per day.

Of course, following the plan to the letter isn't always possible. On a cramped overnight flight, you can't always sleep when the app says to. But Timeshifter suggests that even when you can't sleep, you can still control light exposure. Close the window shade, wear an eye mask, or put on sunglasses. That's the core of the method.

If you're curious, you can try Timeshifter for free on your first trip. After that, it's $9.99 per individual plan or $24.99 per year for unlimited access. Some airlines and hotels, including United Airlines' Premier 1K and InterContinental Hotels, now offer Timeshifter plans as part of their booking experience.

Jet lag isn't just about sleep. It affects your appetite, body temperature, and immune system. That's why a light-based approach can be so effective. So next time you're planning a long-haul trip, consider ditching the sleeping pills and downloading an app that uses the sun instead. And if you're worried about other travel disruptions, check out our story on a text message that grounded a United flight or Copenhagen Airport's new security checks. For more travel tips, see our guide on Europe's coolest winter hotels.