As a fitness professional in New York, I have seen first hand the negative effect that little or low-quality sleep has on my clients. After a couple of weeks of poor rest, my clients invariably develop a nasty cold that puts them out of commission for days, and sets them off track towards good health, success at work, weight loss and fitness, and an active social life. Unfortunately, even if you feel energized after sleeping only a few hours per night, you still make yourself vulnerable to illness. In the study, 77% of the participants reported that they felt rested during the majority of the duration of the study, regardless of their sleep volume. However, the fact remained that if they were sleeping fewer than 8 hours a night, they were significantly more susceptible to developing a cold, despite feeling rested.