John Updike, in his short story “Plumbing,” summarized human nature thusly: “We think we are what we think and see when in truth we are upright bags of tripe.” This is a tragic fact that we spend most our lives trying to forget. Although we like to imagine ourselves as the driver – our consciousness is in full control – that belief is a lovely illusion. In reality, we are mere passengers aboard the body, strapped to a fleshy engine that is driving us.
Consider the orexin system. Secreted by a small cluster of neurons in the hypothalamus, orexin is a neuropeptide that regulates an astonishing array of mental properties, from sleepiness to hunger. People with chronically low levels of orexin suffer from narcolepsy and obesity; many also have cataplexy, which occurs when the experience of strong emotions triggers a sudden weakening of skeletal muscles. (Laughter makes them go limp.) Studies have shown that injecting mice with orexin increases metabolism, largely because it makes the animals more active. The reverse is also true: low levels of orexin make people feel rundown and tired. This helps explain the mechanics of sleep deprivation, as keeping monkeys awake for extended periods all but silences their orexin cells. (However, studies show that the exhaustion can be quickly cured with an injection of the peptide.) In many respects, orexin acts like an internal gas pedal, as even slight twitches in the system can dramatically shift levels of activity.