Not just 'how much' but 'how evenly': Sleep regularity linked to cognitive function

The journal Sleep Medicine published a paper by a team from the University of Tsukuba (Japan) showing that the more regular a person's sleep-wake schedule is, the better their overall cognitive functioning. At the same time, the concentration of the key "neurotrophic" protein BDNF (brain-derived neurotrophic factor) does not behave linearly: it is highest in people with moderate sleep regularity and lower in both very "ragged" and extremely "iron" sleep schedules. The authors cautiously suggest that maintaining a balanced regularity may also be useful in the context of preventing Alzheimer's disease.

Background of the study

In recent years, attention has shifted from a simple “norm” for sleep duration to more subtle characteristics of the regime – the stability of the time of falling asleep and waking up. Even with the same 7-8 hours of sleep, people with a “jumpy” schedule more often report daytime sleepiness, decreased attention, and mood problems. Objective metrics of regularity have appeared, such as the Sleep Regularity Index (SRI), which shows how much your hours of sleep and wakefulness are repeated from day to day. This approach allows us to separate the influence of “how much I sleep” from the influence of “how rhythmically I live”, which is especially important for assessing cognitive functions.

Disruptions in circadian rhythms have long been linked to memory loss, slower information processing, and increased risk of neurodegenerative conditions. The mechanisms are multilayered, ranging from desynchronization of the internal “clocks” of the cortex and hippocampus to disruptions in the hormonal and metabolic signals that prime the brain for learning during the day and recovery at night. In population studies, it is difficult to separate regularity from confounding factors such as age, activity level, sleep deprivation, and chronic disease, so objective sensors and sophisticated statistical models are needed.

A special place is occupied by BDNF - a neurotrophic factor that supports the survival of neurons and synaptic plasticity. Its level fluctuates under the influence of sleep, stress and physical activity and is often considered a "biomarker of learning ability" of the brain. However, most data describe linear relationships (the better the sleep, the higher the BDNF), while in real life the optimum of plasticity often lies between the extremes - not in chronic chaos of the regime, but not in an excessively "concrete" routine.

Against this background, it has become important to test how exactly sleep regularity - measured by actigraphy, not self-reports - correlates with cognitive performance and BDNF levels in a real population. The age context is also important: in middle-aged and elderly people, cognitive functions are more sensitive to circadian desynchronization, and preventive recommendations (sleep, light, activity) must be accurate enough to be applied in everyday life. It is this “hole” between physiological mechanisms and practical prevention that the work under consideration seeks to close.

How the study was conducted

The analysis included 458 adults (mean age 65 years; 51% women) who participated in the Tsukuba Happiness Life Study in 2023-2024. Sleep regularity was measured objectively using the Sleep Regularity Index (SRI) based on continuous 7-day actigraphy (bracelet on the non-dominant hand). Cognitive functions were assessed using an extended test battery; in a subgroup of 232 people, serum BDNF was additionally measured. The authors then built multivariable regression models taking into account age, gender, and total sleep duration.

What is SRI - a short list

• SRI is an index from 0 to 100 that reflects how likely you are to fall asleep/wake up at the same hours every day.
• High SRI → days are similar in sleep and wake times; low SRI → the schedule “jumps”.
• It's an indicator of rhythm, not "how many hours do you sleep."

Main results

After adjustments for age, gender, and sleep duration, participants in the most regular group (SRI ≈65-86) demonstrated a significantly higher overall cognitive score than those with an irregular schedule (B coefficient = 0.13; 95% CI 0.02-0.24). A nonlinear, “inverted U-shaped” relationship was found for BDNF: the highest levels were found with balanced regularity (halo around SRI ≈60), while BDNF was lower with a very irregular and extremely rigid schedule (for the average group, B = 0.17; 95% CI 0.04-0.30).

Why is this important and what does BDNF have to do with it?

BDNF is one of the main factors of brain plasticity: it supports the survival of neurons, the growth of synapses, and the formation of memory. Usually, an increase in BDNF is associated with better learning and stress resistance. The new work adds an important detail: too much order in the routine may not be as favorable for BDNF as reasonable, “lively” regularity. This is consistent with the current idea that the optimum for plasticity often lies between chaos and excessive routine.

How does this compare to other data?

While sleep disruption and acute deprivation have previously been shown to alter BDNF levels and impair working memory, this new work shifts the focus from “sleep deficit” to the rhythm of daily routines. Together, these provide a coherent picture: both quantity/quality and rhythm matter.

What does this mean for everyday life?

The main practical conclusion is to strive not for a “Spartan regime”, but for a stable but flexible sleep regime:

• Keep your sleep "window" stable (for example, go to bed and wake up at similar times on weekdays and weekends, without sudden shifts).
• Avoid extremes: chronic "chatter" and excessive "iron" discipline without regard for real life - both poles were associated with lower BDNF in this study.
• Collect “rhythmic” anchor habits: morning light/walk, regular meals, moderate daytime activity.
• Monitor the overall duration and quality of sleep, but remember: regularity is an independent factor and cannot be reduced to “sleeping 7-8 hours.”

Important limitations

This is an observational study from a single country; causal inferences are limited. BDNF was measured in a subset, and only once; SRI was assessed over a single 7-day period. Finally, the results describe moderate associations in a sample of middle-aged and older adults—they do not automatically generalize to adolescents or, say, shift workers. The authors themselves emphasize the need for longitudinal and intervention studies.

Where to go next - what I would like to check

• To test whether targeted regularity training improves cognitive performance in at-risk groups (MCI, depressive disorders, post-COVID).
• To find out how chronotype, daily activity and light influence the “optimum” SRI for a particular person.
• To assess whether the window of optimum for BDNF and cognitive metrics is robust over time and in other populations. (See popular explanations and press materials for the paper.)

Conclusion

New research supports the idea that it’s not just the hours of sleep that matter to the brain, but also the rhythm. Better cognitive performance is seen in people with more regular schedules, and the neuroplasticity biomarker BDNF peaks at a moderate, “human” regularity—not too chaotic, but not too “paved over” either. For healthy sleep patterns, it appears that the “sweet spot” is not a figure of speech, but a measurable goal.

Research source: Yue Cao et al. Sleep regularity is associated with cognitive function and shows an inverted U-shaped relationship with serum BDNF. Sleep Medicine (online July 17, 2025), DOI: 10.1016/j.sleep.2025.106688.