Many people like to use their phonesor tablets at night before they go to sleep.However, the light from mobile devices can interfere with sleep.But not all light carries the same risk.Recent studies have shown that within the light spectrum,blue light can have the worst effect on sleep.A study in the UK showed that blue light actuallycan keep sleepy people awake.Tired drivers who were exposed to blue lightbecame as alert as those who drank two cups of coffee.Blue light filters have been developed for mobile devicesto help with sleep issues.Apps are available for Android devicesthat filter blue light on screens.Updates to the iPhone, iPad and Amazon Fire tablethave added blue light filters.Twilight is a free app that filters blue light on Android device screens.It can be set to turn on by itself.Or users can turn it on and off when they want.In the app settings, you can change the intensity of the filter,the color and the time the filter turns on and off.Twilight works across all apps,though some apps may not work fully when Twilight's filter is on.There is a YouTube video that explains Twilight use.Twilight is available free for Android devices in the Google Play store.Apple updated its operating systemfor iPhone and iPad earlier this year.Included in the update is Night Shift, a blue light filter.This feature is available on newer iPhones and iPads,including the iPhone 5s and later,and the iPad Air and later.Night Shift can be set to turn itself on every nightor for the user to turn it on and off as wanted.To turn on Night Shift automatically every night,go to Settings, then Display,then Brightness and then Night Shift.On the Night Shift settings, move the switch to schedule Night Shift.It can be set for Night Shift to go on and off0at specific times, or at sunset and sunrise.In Settings you can also adjust the color of the filter.Night Shift can also be turned on and offin Control Center on the iPhone and iPad.Open Control Center by swiping up from the bottom of the screen.The sun and moon icon there identifies Night Shift.Tap it to turn Night Shift on or off at any time.In an update to Fire tablets at the end of last year,Amazon introduced Blue Shade to filter blue light on screens.Blue Shade is available on more recent Fire tablets.To turn on Blue Shade on a Fire tablet,swipe down from the top of the screen and tap Blue Shade.To adjust the color, go to Settings, then Displayand, finally, Blue Shade.