How your iPad messes with your internal clock

It's almost funny to see all the iPhone and iPad apps designed to help people get to sleep. For many people, those gadgets are the very reason they're not getting the rest they need in the first place.

Some people use them to do work and answer emails all night long. For others, tablets have replaced books and even television. And for many people, they've brought the ability to surf the web not just into the bedroom... but right into the bed itself.

These devices don't just keep you awake while you use them. They can actually stimulate your body in ways that will keep you up long after the gadget has been powered down.

The reason is light, which can have a lingering effect on your body. One new study finds that iPads give off the blue light that your body interprets as a signal to shut down production of the sleep hormone melatonin.

In one set of experiments, volunteers were asked to play with an iPad for two hours. In another, they were asked to play with the iPad for two hours while wearing special orange-tinted lenses that filter out blue light.

Not surprisingly, they produced about 23 percent less melatonin after using the iPad without the light-filtering lenses, according to the study in the journal Applied Ergonomics.

The good news is that there was no effect on melatonin after just an hour -- so you could limit the damage by simply limiting your screen time. And, while you're at it, it would also help to turn down the brightness.

Or you could just turn to something a little more old-fashioned and read books and magazines, which give off no light at all. And if you're having trouble finding the right reading to put yourself to sleep, there are a few medical journals I can recommend.