During a normal day, alert-ness remains relatively stable throughout typicalwaking hours in a healthy rested person. Withsufficient sleep, even circadian (i.e. near-24-h)fluctuations in alertness and more rapid variations(e.g. the post lunch dip) during normal wakinghours are trivial and difficult to detect withoutextremely sensitive tests. Nevertheless, when theenvelope of continuous wakefulness is pushedbeyond about 16 h, most individuals begin to showa substantial slowing of reaction time (RT) andworsening of performance accuracy on tests ofpsychomotor vigilance (Goel et al., 2009), declinesthat continue to worsen as wakefulness is sus-tained throughout the night into the early morninghours.106