PHILADELPHIA – The combination of caffeine and evening naps has only modest positive effects on performance and subjective sleepiness of night shift workers, Paula Schweitzer, Ph.D., reported at the annual meeting of the Associated Professional Sleep Societies.
In a rare field study of night shift workers, 39 participants (28 males; mean age 33 years) completed a 4-consecutive-night crossover study comparing two conditions: an evening nap prior to the night shift on the first 2 nights plus caffeine 300 mg on all 4 nights and placebo without napping on all 4 nights.
Three times during each night (start of shift, midway, and end of shift), participants completed a 15-minute psychomotor vigilance task (PVT) using a handheld PVT electronic device that measured the subject's reaction times to tasks. The study subjects also completed subjective sleepiness and mood tests three times a night, said Dr. Schweitzer, associate director of the Sleep Medicine and Research Center at St. Luke's Hospital, St. Louis.
RT10, or the reaction time of the slowest 10% of the PVT responses, worsened across nights 1–4 for both the nap/caffeine treatment group and the placebo group.
“Alertness and performance did not improve across successive night shifts–as has been found in some laboratory studies. People are not getting used to things [on the night shift],” Dr. Schweitzer said.
But some evidence indicated that the caffeine and naps did help somewhat.
Only the placebo group had a worsening of RT10 from shift start to shift end. Additionally, at shift end, the placebo group's RT10 was worse than that of the caffeine/nap treatment group.
The study also evaluated PVT lapses, which are unacceptably long delays in PVT reaction time. Again, the nap/caffeine group outperformed the placebo group. PVT lapse frequency increased from shift start to end for the placebo group, with no change for the treatment group. But both the placebo and treatment groups had an increase in lapse frequency across nights 1–4, which, in effect, was further proof of Dr. Schweitzer's observation that workers are not getting accustomed to night shift work.
Subjective sleepiness as measured by the Karolinska Sleepiness Scale also indicated some benefit of the caffeine and naps. The placebo group showed increased sleepiness at shift end, but the treatment group did not. There was no change in sleepiness ratings across nights 1–4, Dr. Schweitzer said.
Actigraph-estimated total sleep time for daytime sleep did not differ between the two groups.