Effects of sleep deprivation on naval seamen: II.  Short recovery sleep on performance. 
Twenty male naval volunteers, aged 18 to 20 years, with 12 to 14 years of education, underwent a total sleep deprivation experiment on board a Republic of Singapore Navy landing ship in the South China Sea for a period of 42-102 hours.  The sleep group comprised eight volunteers who dropped out at the 44th-46th h of the experiment and were randomly assigned to a 2 or 4 h sleep regime.  The rest served as sleep-deprived controls.  Neurobehavioural performance tests, profile of mood state and the Stanford Sleepiness Scale were applied 6 hourly starting from 6.00 am on the first experimental day.  No thresholds were observed in the performance of tests related to manual tasks and subjective feeling, including naval tasks, mood and sleepiness scale during the sleep deprivation experiment.  However, thresholds were observed in the performance of tests requiring cognitive and perceptive skills, including the grooved peg board, trail making, sea-shore rhythm, addition, digit span, digit symbol, flicker fusion and dynamometer tests.  Performances in these tests were observed to deteriorate only after approximately 30 h of sleep deprivation.  The Z score for the non-threshold tests (Z-N) deteriorated from -0.01 at the start of the experiment to 1.25 at the 42nd h of the experiment just before the imposed sleep and improved to 0.81 at the 48th h of the experiment just after the imposed sleep; and the Z score for the threshold tests (Z-T) varied from -0.07 at the start to 0.49 just before sleep (at the 42nd h) and to continuously deteriorate to 0.83 just after sleep (at the 48th h).(ABSTRACT TRUNCATED AT 250 WORDS)