Being tired all the time might mean there's something more going on than missing out on a few zzz's. In fact, it's estimated that 3 million people around the worls aren't just tired, they have narcolepsy, a disorder that disrupts your normal sleep-wake cycle and makes you feel excessively exhausted, even to the point of falling asleep mid-activity. And, many of those people will be undiagnosed.

Here are some signs to look out for:

When feeling tired isn't normal