Campbell and Murphy (1998)

• 15 sleeping participants were woken at various intervals throughout the night
• 
  

  A light pad was shone onto the back of their knees
• This resulted in irregular sleep-wake cycles for the participants (as high as three hours deviation for some of them)
• The researchers concluded that light is a key exogenous zeitgeber - even when it shines on the back of the knee rather than being detected by the eye