So how important is overnight care for hair? "When we look at the processes in our body, there is a general consensus that at nighttime the body tries to repair itself and do all the maintenance overnight," said Steven Shiel, L'Oréal UK 's director of scientific affairs. "When it comes to hair, it isn’t as clear because hair grows relatively slowly and there aren’t many studies to show that it really grows more overnight than it does in the day, but there is lots of anecdotal evidence," Steven continued. "If your body is working on its internal mechanisms at this time, then it is likely to focus on the hair, too. In fact, scientists are now starting to use samples of hair to measure people’s body clocks, by looking at how the hair follicle is reacting."