[Eduardo]

Eduardo asked the Naked Scientists:
   
Why does a lack of sleep cause dark circles under the eyes?

What do you think?

[Magnus W]

Here is my Guess/theory, When you are sleep deprived your blood presure fals, because of low adrenalin and high melatonin levels. To keep bloodpresure from faling to low your body reduces blood flow to periferal tisuses and your skin, the reduced blood flow couses the skin to look pale or blue (just like your lips get blue when you are hypothermic)