For night shift workers, finding optimal sleep habits often happens by trial and error. Now, science may have delivered a convenient answer.

Researchers at Hiroshima University recently analyzed a trove of studies on night shift naps. In the end, the optimal nap was a lot more than 40 winks. It actually involves two naps, one about 90 minutes, followed by a quick 30-minute shut-eye during work shifts up to 16 hours.

For night shift workers, fatigue and drowsiness set in when the real-world clocks used at work conflict with the body’s internal clock.

In Japan, nurses can sleep up to two hours during long shifts. The researchers wanted to know just how much sleep and how often was ideal. To do that, they re-examined past experiments with a broader eye, including separate studies that assessed alertness and cognitive performance during shifts that involved no naps, one nap and two naps.

Those who took a longer, single nap about eight hours into their shift didn’t fare so well. They experienced drowsiness about four hours after waking from their naps. Those who scheduled two naps, including a first, longer one about seven hours into their shift and another about 10 hours in, did better at staving off drowsiness.

The study also found that nap timing is crucial on long work shifts: The later it is taken, the more potent it is at warding off exhaustion and sleepiness. But waiting too long for that nap could interfere with a worker’s focus as the desire for sleep intensifies.

For those on the night shift, remember to seek out some shut-eye before your body starts trying to shut down.