We know when we are sleepy: subjective versus objective measurements of moderate sleepiness in healthy adults.

The ability to monitor one's sleepiness has obvious implications for safety critical procedures. Laboratory findings indicate that we may be poor at doing this compared with objective measurements (e.g. reaction times (RT)). However, the respective testing situations usually differ, to favour objective measures. These typically entail longer test durations with less distractions; both factors facilitate sleepiness. Using the Karolinska Sleepiness Scale (KSS) we compared subjective responses with RTs, in 2 min epochs, over 10 min periods in identical quiet settings, early afternoon, in 21 healthy volunteers with 5h prior night's sleep restriction. Whereas the initial KSS score was unrelated to 10 min RT, the KSS subsequently showed a similar, significant increase, comparable with RT. Changes in both scores were very significantly correlated. KSS scores indicated that 5 min was an effective 'settling down' period. Participants were good at estimating their sleepiness if presented with a procedure equivalent to that of the objective measure.