Foggy by day and tossing at night. A study suggests this may not be simple fatigue, but a sign that the brain may be shrinking faster.
These changes appeared more often as the “circadian rest-activity rhythm”—the regular 24-hour pattern of alternating activity and rest—became more disrupted. The difference was observed even in people without dementia or cognitive impairment.
A sign that the brain may be shrinking faster
A joint research team from the Johns Hopkins Bloomberg School of Public Health and the U.S. National Institute on Aging found that older adults with weaker or more fragmented circadian rest-activity rhythms showed faster shrinkage in brain regions affected early in Alzheimer’s disease. The findings were published April 14 in the international journal 〈Alzheimer's & Dementia〉.
In this context, a “weak” rhythm means the difference between daytime and nighttime activity is less distinct, while a “fragmented” rhythm means periods of activity and rest are frequently interrupted.
The study included 344 adults ages 50 and older (average age 73) with no cognitive impairment. Participants wore a wrist activity monitor for up to one week to record their 24-hour rhythm and also underwent brain MRI scans.
They were participants in the Baltimore Longitudinal Study of Aging run by the U.S. National Institute on Aging. In this study, the same individuals are assessed repeatedly at roughly one-year intervals to examine changes over time.
The analysis focused on the hippocampus, the surrounding hippocampal region (parahippocampal gyrus), and the amygdala—areas where Alzheimer’s-related damage can appear early.
MRI confirmed differences in the “rate of brain shrinkage”
The results had two main parts. First, at the initial assessment, people with fragmented rhythms already had smaller volumes in the hippocampus, parahippocampal gyrus, and amygdala than those without fragmentation.
Then, about one year later, MRI scans were repeated. People with more stable rhythms showed a relatively slower rate of amygdala shrinkage. In contrast, the more fragmented the rhythm, the more the ventricles expanded.
The ventricles are fluid-filled spaces inside the brain, and they become relatively larger as brain tissue decreases. Ventricular enlargement is interpreted as a sign that brain atrophy is progressing.
This difference was more pronounced with older age. In particular, among those in their 80s and older, the association between maintaining a stable rhythm and preserving the amygdala was strongest.
Professor Adam Spira, who led the study, explained, “Circadian rhythm disruption and neurodegeneration may influence each other in a bidirectional relationship.”
When the rhythm breaks down, the brain may break down too
Because this was an observational study, it cannot prove that circadian rhythm disruption directly causes brain atrophy. However, an association between the two was identified.
In people with dementia, sleep-wake rhythms are often already severely disrupted. This study suggests such changes may appear earlier, even in people without cognitive impairment. It also indicates that brain health may be more closely tied to rhythm consistency than to the total number of hours slept.
The research team proposed two non-drug strategies: keeping a fixed wake-up time and getting morning light exposure.
Keeping wake-up time consistent helps anchor the body clock. Light is a key signal that helps the suprachiasmatic nucleus (the brain’s “central clock,” which receives light input and regulates daily rhythms) stay aligned. When morning sunlight enters the eyes, melatonin (a hormone that promotes sleep) is suppressed and wakefulness begins. Conversely, if this signal is delayed, the entire daily rhythm shifts later.
The habit of checking a smartphone immediately after waking also warrants caution, because artificial light can send the wrong timing signal.
The cost of foggy days
This study suggests that brain changes may begin before symptoms become noticeable.
Feeling foggy during the day and waking frequently at night are not separate issues. They can both stem from a disrupted body clock. Over time, these changes may be associated with structural differences in the brain.
The response needed at this stage is clear. Wake up at the same time every day, get enough morning light to keep the start of your day consistent, and gradually cut back—one by one—on habits that throw off your rhythm.
