There was a graded association with average sleep duration: participants with less than 7 hours of sleep were 2.94 times…more likely to develop a cold than those with 8 hours or more of sleep. The association with sleep efficiency was also graded: participants with less than 92% efficiency were 5.50 times …more likely to develop a cold than those with 98% or more efficiency. These relationships could not be explained by differences in prechallenge virus-specific antibody titers, demographics, season of the year, body mass, socioeconomic status, psychological variables, or health practices. The percentage of days feeling rested was not associated with colds.
Even if there’s a big confound that they haven’t accounted for, this is a large effect. It also goes along with our intuitions and common sense. There are plenty other correlations between less sleep and illness out there, but respiratory illness is a great proximate dampener on productivity for many of us. Also, ScienceDaily.