I’m sure it’s news to no one, but when you are tired it’s really difficult to resist snacking on junk food. For me it strikes hardest at the beginning of an evening work shift; when five o’clock hits I can often be found rummaging in my bag for coins and power walking to the fundraiser chocolates, and even stale cake in the tearoom becomes tempting.
To a certain extent, it makes sense that we snack more when we’re awake for longer; an awake and active body tends to use more energy than a sleeping one. However, research suggests that the extra calories consumed when people ‘tired eat’ far outweigh the energy required to stay awake. In other words, sleep deprived people tend to eat for reward and pleasure rather than out of necessity. With a decrease in average sleep times and a growing body of evidence linking insufficient sleep to an increased risk of obesity, the tendency to binge eat when tired presents a real health problem.
In order to find out more about why we crave highly calorific snacks when sleep deprived, researchers at the University of Chicago investigated the potential impact of the endocannabinoid system. Along with its involvement in areas such as pain, mood and memory, this system is known to play a key role in stimulating reward-based or ‘hedonic’ eating.
The lynchpins of the endocannabinoid system are the ‘endocannabinoids’ themselves, a group of chemicals synthesised by the body that behave in a similar way to the active ingredients of marijuana (hence the reference to cannabis in the name). The system also contains enzymes for endocannabinoid synthesis and degradation, as well as the receptors that both these chemicals and those found in marijuana bind to to cause changes in the body. There are two types of cannabinoid receptors: cannabinoid receptor type one (CB1) found mainly in the brain and spinal cord, and cannabinoid receptor type two (CB2) found on immune cells and tissues. It is the CB1 receptor that has an impact on appetite, and is also responsible for the marijuana high.
The small study conducted at the University of Chicago and published in Sleep involved fourteen non-obese men and women aged 18-30 years who normally slept between 7.5 h and 8.5 h at night. Each subject was kept on a fixed diet (three identical meals per day) and randomly subjected to either four nights of normal sleep (8.5 h) or restricted sleep (4. 5h). On the third night, each patient had their blood levels of the endocannabinoid 2-arachidonoylglycerol (2-AG) measured every hour for 24 hours, and during this period were asked questions about their hunger, appetite and food intake. The day after, participants were treated to a buffet lunch and dinner with unlimited snacks in between, and the caloric breakdown and quantity of food consumed determined. Then around four weeks’ later the participants were exposed to the opposite sleep condition, creating what known as a ‘randomised crossover study’.
The researchers found that for all subjects 2-AG peak levels occurred in the early afternoon and dropped to a minimum overnight. Interestingly however, while the well-rested volunteers had a 2-AG peak at around midday that soon declined, the sleep-deprived peak 2-AG levels were around 33% higher, peaked at around 2 pm and persisted until around 9 pm (which isn’t good, as recent data also suggests a link between late eating and increased weight gain). During this period of elevated 2-AG, the sleep deprived volunteers reported increased hunger and appetite, and were less able to resist fat and protein-filled snacks despite not needing the extra calories.
What these findings suggest is that when people are sleep deprived, it may well be their overactive endocannabinoid systems helping to drive their hunger. And when you think about how cannabis works in the brain, the study findings make sense. When someone smokes marijuana, the THC binds to the CB1 receptors in their brain, which leads to a huge increase in appetite. Similarly, when the body is sleep deprived more 2-AG is produced, which binds to CB1 and makes us hungrier. This means that when we are tired, we get the biochemical equivalent of the ‘marijuana munchies’.
So why have we evolved to get the munchies when we are sleep deprived? In a discussion with The Naked Scientist’s Dr. Chris Smith, lead researcher Dr. Erin Hanlon admitted that they didn’t know. A hypothesis proposed by the researchers is that it may have been protective at some point in our evolutionary history; our ancestors would have had to overeat when food was plentiful to store enough energy for times of famine, but now that food is readily available this response has become maladaptive.
While certainly providing me with an insight into why I’m such a snacker, perhaps the most exciting impact of this study is the implication that endocannabinoids and their receptors could make good targets for anti-obesity drugs. The efficacy of such treatments have already been demonstrated in animal studies; a study published in the Journal of Clinical Investigation in 2003 showed that when the gene for CB1 is deleted in mice they eat less and become leaner. However, given the potential complications associated with interrupting the endocannabinoid system (the CB1 blocker drug Rimonabant was withdrawn from the market because of major psychiatric side effects) a lot more research into this area is required. In the meantime, what I’m taking away from this study is that when I’m tired and hungry, maybe I should think twice about having that fifth biscuit…