I thought this could be measured, e.g. with an EEG, or by measuring the sleeper's rapid eye movement (REM). "REM sleep" is generally when we have dreams. I wonder if these people who don't dream just don't REMEMBER their dreams, or actually don't have REM sleep? I thought that you HAD to have the latter, or else your brain would become exhausted and you could die from it. More likely, is that before this happened you would just collapse from exhaustion and THEN get some REM sleep for sure.