This is a nice study showing cause and effect: 6 hours of sleep or less at night lowers the body's resistance so that the person is more likely to catch a cold virus. From Science Daily:
A new study led by a UC San Francisco sleep researcher supports what parents have been saying for centuries: to avoid getting sick, be sure to get enough sleep. The team, which included researchers at Carnegie Mellon University and University of Pittsburgh Medical Center, found that people who sleep six hours a night or less are four times more likely to catch a cold when exposed to the virus, compared to those who spend more than seven hours a night in slumber land.
Scientists have long known that sleep is important for our health, with poor sleep linked to chronic illnesses, disease susceptibility and even premature death. Prather's previous studies have shown that people who sleep fewer hours are less protected against illness after receiving a vaccine. Other studies have confirmed that sleep is among the factors that regulate T-cell levels.
Researchers recruited 164 volunteers from the Pittsburgh, PA, area between 2007 and 2011. The recruits underwent two months of health screenings, interviews and questionnaires to establish baselines for factors such as stress, temperament, and alcohol and cigarette use. The researchers also measured participants' normal sleep habits a week prior to administering the cold virus, using a watch-like sensor that measured the quality of sleep throughout the night.
The researchers then sequestered volunteers in a hotel, administered the cold virus via nasal drops and monitored them for a week, collecting daily mucus samples to see if the virus had taken hold. They found that subjects who had slept less than six hours a night the week before were 4.2 times more likely to catch the cold compared to those who got more than seven hours of sleep, and those who slept less than five hours were 4.5 times more likely.