Getting enough good sleep is one of the best ways to stay fit and healthy. So hear this: The more screen time you put in on your smartphone, computer, or tablet—all day, not just near bedtime—the poorer and shorter your sleep will be, say UCSF scientists, who analyzed data on 600-plus subjects.

The culprit: Blue light emitted by screens, which hinders the sleep hormone melatonin.

To fight the blues, block rays from your phone or computer with an app from justgetflux.com or twilight.urbanandroid.org.