After an ample meal, all of us are familiar with the sometimes irresistible urge to take a nap. However, obeying this urge while forking down dinner,lunch or tea could quickly turn into an inconvenient complaint, generally diagnosed as `excessive daytime sleepiness'. This sort of illness represents not only a growing health problem associated with our hectic lifestyles but also offers new insights into the link between sleep and obesity.

To specifically address the issue of sleep regulation and body condition in mammals, Joe Jenkins and his colleagues from the Department of Psychiatry in Pennsylvania, USA performed an appetising experiment on mice. They measured electroencephalogram (EEG) and electromyography (EMG) activity in the brains of black mice while they were awake and during sleep. Before starting the experiment, the team recorded baseline sleep patterns while feeding regular lab chow to all the mice. Next, they divided the mice into two groups; one that was fattened with high fat food, and another, the control group of mice,which kept their figures by ingesting normal food. As the scientists were interested in the sleep behavior of obese and thin mice, they continuously monitored the animal's brain activity while asleep, and awake over 2, 4 and 6 week intervals. The animal's sleep and awake states were scored by quantifying brain activity wavelengths from the EEG and EMG recordings and attributing them either to non-rapid eye movement sleep (NREMS), rapid eye movement sleep(REMS), or to the awake behavioural state. In addition to that, Jenkins and his team measured the animal's food intake and body mass during the course of the experiment.

Consistent with observations in well-fed humans, the researchers found that in the experimentally fattened mice, the animal's sleepiness increased in parallel with their weight gain. Simultaneously, their wakefulness declined after 2, 4 and 6 weeks of gorging. Interestingly, the mice's REMS and NREMS sleep patterns were affected differently by weight gain; episodes of NREMS significantly increased, whereas their REMS remained unaltered. By correlating the increase of NREMS with both the increase in body weight and energy intake,Jenkins and his co-workers found out that the weight gain itself is responsible for increased sleepiness, not the net food intake. They explain that the underlying metabolic process for this finding remains to be uncovered and propose taking a closer look at the expression of inflammatory cytokines such as tumor necrosis factor (TNF) and interleukin-6 (IL-6). As obesity is a chronic inflammatory state, these two cytokines, which either circulate in plasma (TNF) or are found in fat (IL-6), are both elevated in obese individuals, and may regulate sleep or are possibly involved in sleep disorders. Altogether, Jenkins' study indicates that obese animals experience an increased pressure to sleep resulting in a reduction in their wakefulness,especially during the active phase of the day. So those of us who dreamt of catching forty winks before reading this are advised to sleep on it before taking that snooze.

Jenkins, J. B., Omori, T., Guan, Z., Vgontzas, A. N., Bixler, E. O. and Fang, J. (
2006
). Sleep is increased in mice with obesity induced by high-fat food.
Physiol. Behav.
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