Marie-Pierre St-Onge, PhD: There’s not a set number of hours that’s considered optimal for everyone. But in general, scientists agree that 7 to 8 hours is a healthy amount of sleep. When we look at studies linking obesity for example and sleep duration, we find that individuals that sleep less than 7 hours per night tend to be more obese than individuals who sleep 7 to 8 hours per night. And then when you get into the long range also, there seems to be an increase in the prevalence of obesity in those who sleep 9, 10 hours per night. So, 7 to 8 hours per night seems to be a sweet spot.