First, because it will help you keep happy memories. Sleep, it seems, protects positive memories (and also negative ones), which has important implications for the treatment of post-traumatic stress disorder (PTSD).

To test how sleep affects positive memories, Rebecca Spencer, from the University of Massachusetts, USA, had one group of patients sleep overnight and another group remain awake. Both groups viewed images of positive items, like puppies and flowers, and neutral items, like furniture or dinner plates. The researchers then evaluated the participant's memories of the images.

They found that "sleep enhances our emotionally positive memories while these memories decay over wake time," Spencer says. "Positive memories may even be prioritized for processing during sleep." But while people remembered the positive images more than the neutral ones, their emotional response to the positive images did not change over sleep versus wake. The results could have significant implications in treatment of PTSD, as using wakefulness could have the unintended effect of degrading positive memories in addition to negative ones.

Can lack of sleep make you fat?
A new review article by Dr. Kristen Knutson, from the University of Chicago, examined experimental and observational data from sleep restriction studies. The research explored how a lack of sleep can impact appetite regulation, impair glucose metabolism, and increase blood pressure. Their results revealed cross-sectional associations between getting fewer than six hours of sleep and increased body mass index (BMI). Inadequate sleep impacts secretion of the signal hormones ghrelin, which increases appetite, and leptin, which tells the body you are satiated. This increase in hormone signalling can result in increased food intake.

"These findings show that sleeping poorly can increase a person's risk of developing obesity, diabetes, high blood pressure or heart disease," concluded Dr. Knutson. "Future research should determine whether efforts to improve sleep can also help prevent the development of these diseases or improve the lives of patients with these conditions."

Tofu burger or big Mac? MRI scans show how sleep loss affects the ability to choose proper foods
In another study, led by Dr. Matthew Walker, from the University of California, Berkeley, USA, sleep loss was found to impair the higher-order regions in the human brain: the right anterior insula and the dorsal anterior cingulate, parts of the frontal lobe where food choices are made. Their findings may help explain the link found by previous studies between sleep loss and obesity. Functional magnetic resonance imaging (fMRI), was done on one group of patients after a normal night's sleep and on a second, sleep-deprived group. Both groups then answered questionnaires rating their desires for various food items, shown to them while inside the fMRI scanner. Sleep deprivation decreased the correlation between food desire and taste ratings. The study suggests that sleep loss may impair the higher brain functions, critical for making appropriate food choices, rather than affecting activity in deeper brain structures that react to basic desire.

Articles:
Processing of Emotional Reactivity and Emotional Memory over Sleep. DOI 10.1523/JNEUROSCI.2532-11.2012
Does inadequate sleep play a role in vulnerability to obesity? DOI DOI: 10.1002/ajhb.22219