You know the classic signs: You're dragging at work, you're yawning all day, you're sluggish and you swear you can actually hear your bed calling your name. You need to sleep.

On the days when a fourth cup of coffee is masking the most obvious signs, you might think you've won the battle. But don't kid yourself. There are some tell-tale, though surprising, signs that your body is craving shuteye, even if you might chalk them up to other factors.

In fact, about a third of American workers, or roughly 40.6 million adults, get fewer than six hours of sleep each night, HuffPost reported in April

Getting less than the optimal seven to nine hours recommended by the CDC can result in serious health issues, like heart problems and obesity. But it can also disturb your day-to-day behavior in some more subtle ways.

Click through the slideshow below for some of the more surprising signs you need to sleep. Then tell us in the comments which ones you've noticed.

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  • You're Ravenous

    If you find yourself hungry all day (and not because you skipped breakfast or have recently amped up your gym routine) it might be because you've been skimping on sleep. Research presented at the 2010 meeting of the Society for the Study of Ingestive Behavior linked little shuteye with <a href="http://www.huffingtonpost.com/2012/07/10/sleep-hunger-deprivation-_n_1659954.html">higher levels of the hormone ghrelin</a>, the same one that triggers hunger, HuffPost reported. This uptick in the hunger hormone seems to lead to not only increased snacking, but also a hankering for <a href="http://www.sciencedaily.com/releases/2004/12/041206210355.htm">high-carb, high-calorie foods</a>, according to a 2004 study, which may help explain why people who don't get enough sleep are at a greater risk of obesity.

  • You're Weepy

    Ever find yourself tearing up over an embarrassing TV commercial? While women might be quick to blame PMS, it could be a lack of sleep sending your emotions into overdrive. A 2007 study found that sleep-deprived brains were <a href="http://usatoday30.usatoday.com/news/health/2007-10-22-sleep-deprivation-brain_N.htm">60 percent more reactive</a> to negative and disturbing images, <em>USA Today</em> reported. "It's almost as though, without sleep, the brain had reverted back to more primitive patterns of activity, in that it was <a href="http://berkeley.edu/news/media/releases/2007/10/22_sleeploss.shtml">unable to put emotional experiences into context</a> and produce controlled, appropriate responses," Matthew Walker, senior author of the study, said in a statement.

  • You're Forgetful Or Unfocused

    You might be tempted to blame your trouble focusing on your age or stress or your overflowing email inbox, but a lack of sleep could be the true culprit. Too few hours in dreamland has been linked to a <a href="http://www.webmd.com/sleep-disorders/excessive-sleepiness-10/emotions-cognitive">whole host of cognitive problems</a>, like difficulty focusing and paying attention, confusion, lower alertness and concentration, forgetfulness and trouble learning, WebMD reports. So next time you find yourself forgetting where you put your keys, consider how much sleep you got last night.

  • You Can't Shake That Cold

    If you keep coming down with the sniffles -- or can't seem to kick that never-ending case -- you might want to assess your sleep schedule. A 2009 study found that people who sleep fewer than seven hours each night have almost <a href="http://articles.latimes.com/2009/jan/17/science/sci-sleep17">three times the risk of catching a cold</a> than people who slept for at least eight hours, the <em>LA Times</em> reported.

  • You're Clumsier Than Usual

    First you knock the alarm clock off the dresser, then you spill the milk as you're pouring your cereal, then you stub your toe on the way out the door -- you've become a klutz overnight. Researchers don't know exactly why, but sleepy people seem to <a href="http://www.prevention.com/amisleepdeprived/list/5.shtml">"have slower and less precise motor skills,"</a> Clete Kushida, M.D., Ph.D., director of Stanford University Center for Human Sleep Research told <em>Prevention</em>. Reflexes are dulled, balance and depth perception can be a little wonky and since you may also have trouble focusing, reaction time can be slowed, meaning you can't quite catch the egg carton before it hits the floor.

  • You've Lost That Lovin' Feeling

    If you or your partner just can't get in the mood, and stress or an underlying health problem isn't to blame, you might want to spend some extra time between the sheets -- sleeping. Both men and women who don't get their 40 winks experience a <a href="http://www.webmd.com/sleep-disorders/excessive-sleepiness-10/10-results-sleep-loss">decreased sex drive</a> and less interest in doing the deed, WebMD reports. A lack of sleep can also <a href="http://www.everydayhealth.com/erectile-dysfunction/causes-of-low-libido.aspx">elevate levels of cortisol</a>, the stress hormone, according to Everyday Health, which doesn't help in the bedroom either.

For more on sleep, click here.

Flickr photo by ohmeaghan