In your human factors and limitations book on page 99 you state that there are only 4 stages of sleep. Online aviation theory aswell as other sources suggest there are 5.
Could you confirm please ?
It all depends on whether you classify REM (rapid eye movement) as a separate stage of the sleep cycle or as a characteristic of stage 4 sleep. The research I did initially classified REM as being part of stage 4 sleep. I note that a Google search for 'stages of sleep' shows many publications that still indicate four stages with REM included in stage 4 sleep. Can you advise the other publications you have found the indicate five stages. Thanks for your feed-back. It is appreciated.
It seems that different sources have different opinions as two stages are quite similar and people count them as one. However it looks like technically it's 5 stages.
https://sleepfoundation.org/sites/default/files/SleepWakeCycle.pdf
See page 9
Thanks
