Skin serves an important immune function, protecting your internal organs and acting as a barrier to infection and bacteria. It also acts as an indicator to your psychological state – it flushes when you're flustered, blinks when you're nervous and sweats when you're worried.
In the same way, stress and tiredness show up in grey, dull skin. Which is another reason why it's important to invest in your happiness, take measures to reduce or at least balance stress with relaxation techniques and to try to get a good night's sleep as often as possible. The results really do show inside and out.