Nothing feels quite as good as sleeping in... especially after a few too many late nights in a row.

But while it's great to catch a few extra winks every now and then, too much sleep can actually be every bit as bad for you as too little over the long run.

And now, a new study finds that both undersleepers and oversleepers alike could be snoozing their way to an early mental decline.

British researchers looked at data on 5,431 office workers who were between the ages of 35 and 55 when the study (the famous Whitehall II) began in 1985. These volunteers were quizzed about their sleep habits once between 1997 and 1999, and again between 2003 an 2004.

Then, they were given cognitive tests.

The women who did best on those tests got an average of seven hours of sleep a night, while second-best went to those who got six.

For men, the precise number of hours didn't matter so much--they did the same as long as they got between six and eight hours a night.

And when those sleep habits changed, the real problems began.

The researchers found that volunteers who went from normal sleep during the first survey to too much or too little in the second did so poorly on cognitive tests that it was as if they had suffered between four and seven extra years of aging.

The researchers say those who got too little sleep had lower scores in reasoning, vocabulary, and global cognitive status.

Those who got too much, on the other hand, flunked up to six cognitive tests when compared to those who had the same sleep habits all along.

That's bad enough--but that's not even close to the worst problems that have been linked to poor sleep habits. Other studies have found that people who sleep too little or too much face heart problems, stroke, and even an increased risk of death itself.

One recent study found that men with insomnia were four times more likely to die during the study period than men who slept well. I have all the details here-- along with a few tips that can help anyone sleep better.

Consider this your wake-up call.