Flies in deep stage sleep (IMAGE) Northwestern University Caption Proboscis fully extends (red arrow) and then immediately retracts during proboscis extension sleep, which is similar to slow-wave sleep in humans. Credit Ravi Allada/Northwestern University Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.