Obstructive sleep apnea (OSA) can cause excessive daytime sleepiness, and many patients with OSA are obese. However, some obese patients with OSA are still sleepy despite regular treatment with CPAP. Several studies have linked excessive sleepiness with obesity in patients without OSA. Clinically, I see both conditions often enough, and sleep researchers do not understand exactly why these patients are sleepy. Drs. Panossian and Veasey at the University of Pennsylvania published a review article in the May edition of the Journal Sleep about this topic.
The article describes how patients with OSA who undergo bariatric surgery sometimes have a dramatic improvement in subjective sleepiness a few weeks after the surgery. I have seen this in my clinic as well, even though the patients may still have OSA. Again, the reason for the improvement in sleepiness is not known.
Food can affect sleepiness too. The article describes a complicated system of hormones and other bloodstream chemicals that are released in response to food ingestion that can inhibit our alertness.
There is a group of cells in a deep part of the brain called the hypothalamus, and these cells are involved in maintaining our wakefulness. In narcolepsy, these cells die off, resulting in excessive daytime sleepiness. Some studies are showing that these cells can stop functioning normally in the context of obesity, and this could contribute to sleepiness.
The authors conclude by stating that in patients who are obese, OSA is a common enough disorder that it should be ruled out if the patient is sleepy. If there is no OSA to explain the sleepiness, than it could be due to a combination of the obesity itself and dietary influences. How we treat that sleepiness is still up in the air - for now it appears that weight loss may improve alertness. Easier said than done.