In a carefully controlled two-part experiment, scientists began by collecting nightly sleep data on 164 healthy individuals for 1 week. Participants were asked to record the times at which they went to bed and woke up. They also wore small watchlike devices that use a technique called wrist actigraphy to monitor movement (much like a Fitbit tracks activity) while they slept. Aric Prather, lead author of the study and a sleep researcher at UC San Francisco, says that he and his colleagues associate the wrist actigraphy data with being awakeif during a reported sleep period, the wrist band records movement, they take that as an indication of wakefulness, and subtract the time spent moving from the hours asleep.Then came part two: the cold infections. Scientists quarantined participants in a hotel and gave them nose drops containing rhinovirusthe virus responsible for the common cold. They then closed off the hotel floor for 5 days, letting the hosts' immune system do the rest. To ensure the most accurate results, researchers drew participants' blood before the viral exposure to test for levels of rhinovirus antibody, a defensive agent in the immune system that recognizes and attacks rhinovirus. If they found high, preexisting levels of the protective protein, they removed the participant from the study so that prior immunity would not bias the infection rates of the group.

In order to officially register as "sick," participants had to exhibit one "objective sign of illness" and one other immune response. Signs of illness revolved around mucus production. After viral exposure, scientists collected used tissues daily and, essentially, weighed the snot. Ten grams or more counted as a sign of illness. They also looked at congestion. The researchers dripped a harmless dye into participants' noses and waited to see how long it took to reach the back of their throats; longer than 35 minutes tallied as a sign of illness. A valid immune response required one of two things: A mucus sample flushed from a participant's nasal passage had to show signs of viral replication or blood work needed to show new levels of the rhinovirus-fighting antibody.Of the 164 participants, 124 received the actual virus instead of the control, and 48 of them got sick. By checking the sleep duration of the sick participants, researchers report in the current issue of SLEEPthat individuals who slept fewer than 5 hours a night were 4.5 times more likely to get sick than those who slept 7 hours or more. Those who slept 5 to 6 hours were 4.2 times more likely to get sick, but those who slept 6 to 7 hours per night were at no greater risk of catching the cold than those who slept 7 hours or more, suggesting that there's a sleep threshold for potent immune defense.

Find the experimental units/subjects and the variable, the population,the sampling method , any treatments imposed on the subject, any biases that might have influenced the results, and what that influence would be