Stop! Put down that turkey sandwich and back slowly away from the fridge. Your nocturnal noshing may not only be bad for physical health, it could also be detrimental to learning and memory, according to the latest neuroscientific research.
A substantial amount of scientific study has already shown that late-night culinary habits can contribute to the development of conditions such as obesity or type 2 diabetes. Now a team at the University of California, Los Angeles has looked at the ways eating late can affect the brain.
Nearly all plants and animals display numerous biological processes that oscillate over the course of the day. For humans, these cyclical processes, called circadian rhythms, influence when we sleep, wake up, eat meals and even when we are physically strongest. “The goal of circadian clocks is to align our internal biology with the 24-hour environment,” says Ravi Allada, chair of the department of neurobiology at Northwestern University. “The environment is able to reset our clocks so that we are kept in sync with what is going on around us. And the most prominent synchronizer is light.”
Scientists originally believed that circadian behaviors were controlled exclusively by the brain’s “internal clock,” located in a region called the suprachiasmatic nucleus (SCN) that is directly modulated by light hitting the retina. However, further research has shown that other regions of the body—such as the hippocampus, a brain area important for regulating memory—contain timekeeping mechanisms of their own that may respond to stimuli other than light.
When our internal rhythms fall out of sync with the external environment, as in the case of jet lag, we experience impairments in physical health as well as cognitive functioning. “One of the consistent things we see in people who have disruptions in their circadian rhythms is memory deficits,” says Christopher Colwell, a professor of psychiatry at the UCLA School of Medicine and co-author of the study. For years, his team has investigated how disruptions to the sleep/wake cycle have an impact on learning and memory.
In the current study, which has yet to be published, the research team investigated how the timing of meals affects biological rhythms and behavior. Unlike the transient misalignment associated with jet lag, Colwell and his team were “interested in looking at sleep disruption that is chronic, because so many people in our society are dealing with this issue,” says Colwell. This is in part because, with the advent of artificial light, workdays have been extending later into the evening, resulting in increasingly delayed dinners.
Using mice as subjects, the researchers created two-week-long feeding schedules that were either aligned or misaligned with the animals’ natural circadian cycles. Because mice are nocturnal, misaligned meals were given during the day and aligned meals were given at night.
The team observed that this simulated “late-night snacking” had striking consequences for a variety of behaviors. “Just by feeding them at the wrong time, we get this disruption of the entire system,” says Colwell.
Though both groups slept for the same total amount of time, the misaligned eaters showed reduced sleep during the day and increased sleep at night, compared to their aligned counterparts. These alterations were accompanied by an increase in overall activity levels during the day (when mice are normally sleeping) and decreases in activity during the night (when mice are normally awake). Thus, misaligned eating disrupted the cyclical timing of sleep.
Curious if these changes were related to misaligned internal timekeeping, the team probed the cellular circadian processes in various organs throughout the mouse body. Their results showed that although the central timekeeper was ticking smoothly, the hippocampus, liver and adrenal glands had all shifted functionality due to misaligned eating behavior.
“We showed that under these eating conditions, some parts of the body, especially the hippocampus, are completely shifted in their molecular clock,” says Colwell. “So the hippocampus, the part of the brain which is so essential for learning and memory, is actually following when the food is available.” That means the memory function of the brain is affected by food, and late eating produces an internal misalignment in the body.
The researchers next sought to measure the effects of misaligned eating on the subject’s ability to learn and remember. They tested the mice’s ability to recognize new objects in their cage and also assessed their ability to remember the pairing of a sound tone with a painful shock—tasks that are known to depend on a properly functioning hippocampus. On both tasks, the misaligned eaters showed impaired learning and memory ability as compared to the aligned eaters.
In yet another series of experiments, the scientists also showed that midnight eaters expressed significant cellular deficiencies in something called synaptic plasticity, a process that is thought to be fundamental for our ability to form new long-term memories.
The implications of this work aren't all dire. The research team is now highly interested in investigating the underlying cellular mechanisms behind these phenomena, how different diets—say, high fat versus low fat—affect learning and memory and whether mealtime can be manipulated as a therapy to help realign dysfunctional circadian clocks.
“So many people, either because of work or because of diseases of the nervous system, are under situations where their biological clock is chronically disrupted,” says Colwell. “We think that we are uncovering a tool that we can use to either strengthen or weaken the clock, just by controlling when a person eats.”