Circadian phase and sex shape swimming exercise responses and stereotyped behaviors in mice

This study demonstrates that circadian phase and sex interact to differentially modulate swimming behaviors, physiological stress responses, and post-exercise emotional outcomes in mice, highlighting the critical need to account for these factors in preclinical exercise research.

The Gist

Imagine your body is like a high-tech smartphone. It has a built-in "operating system" called your circadian rhythm (your internal clock) that tells it when to be awake, when to sleep, when to eat, and when to exercise. Just like a phone battery drains faster if you try to run heavy apps at the wrong time, your body reacts differently to exercise depending on when you do it.

This study is like a scientific experiment where researchers took a bunch of mice (our furry little test subjects) and asked: "Does it matter if we make them swim for exercise during their 'work hours' or their 'nap time'?"

Here is the story of what they found, broken down into simple concepts:

1. The Setup: Two Different Time Zones

The researchers split the mice into two groups, like two different time zones:

• The "Regular" Group (REG): These mice lived on a normal schedule. The lights turned on at 7 AM (their morning) and off at 7 PM (their night). Since mice are nocturnal (night owls), their "work day" is actually the dark hours. So, when the researchers made them swim at 10 AM, it was mid-nap time for these mice.
• The "Inverted" Group (INV): These mice had their schedule flipped. The lights turned off at 10 AM and on at 10 PM. For them, 10 AM was their morning (their active time).

Both groups were forced to swim in a pool for 20 minutes every day for three weeks. The researchers watched them like hawk-eyed coaches, counting every move.

2. The Swimming Pool: Panic vs. Chill

When you put a mouse in a pool, it has two main ways to cope: Fight (swim frantically, try to climb out, bump into others) or Flight/Freeze (float, conserve energy, just keep its nose above water).

• The "Nap Time" Swimmers (REG): When the mice were forced to swim during their nap time, they were stressed.
  • Males: They went into "panic mode." They tried to climb the walls, bumped into each other, and fought the water.
  • Females: They went into "shutdown mode." They floated more, barely moving, trying to save energy. They were the most exhausted group.
• The "Morning" Swimmers (INV): When the mice swam during their natural active time, they were much calmer. They didn't panic as much, and the differences between boys and girls disappeared. They just swam like normal.

The Analogy: Imagine being forced to run a marathon at 3:00 AM when you are dead tired (REG) versus running it at 7:00 AM when you are fresh (INV). The 3:00 AM runners are going to be grumpy, clumsy, and desperate to stop. The 7:00 AM runners are just doing their job.

3. The Aftermath: Weight and Stress Hormones

After the swimming sessions, the researchers checked the mice's weight and their "stress hormones" (corticosterone, which is like the mouse version of human cortisol).

• The Nap Time Penalty: The mice that swam during their nap time (REG) suffered the most.
  • Weight: The female mice actually lost weight. They were so stressed and burned so much energy that they couldn't keep it on.
  • Stress: Their stress hormone levels skyrocketed. It was like their bodies were screaming, "We are under attack!"
• The Morning Swimmers: The mice that swam during their active time (INV) didn't lose weight, and their stress hormones didn't spike as much. Their bodies handled the exercise like a normal workout.

4. The "Open Field" Test: Anxiety After the Swim

The next day, the researchers put the mice in a big, empty box with a bright center and dark corners. This is a classic test to see if an animal is anxious. Anxious mice hide in the corners; confident mice explore the center.

• The Surprise: The mice that swam during their active time (INV) actually became more curious and explored more, especially the females.
• The Anxiety: The mice that swam during their nap time (REG), specifically the females, became very anxious. They refused to go into the center of the box. It was as if the "wrong-time" exercise made them feel unsafe and scared the next day.

The Big Takeaway

This study is a wake-up call for how we think about exercise and biology.

1. Timing is Everything: Exercise isn't just "good" or "bad." It depends entirely on when you do it relative to your body's internal clock. Doing intense exercise when your body wants to sleep can turn a healthy habit into a massive stressor.
2. Men and Women (Mice) React Differently: The study showed that male and female mice reacted very differently to the "wrong-time" exercise. The females were hit the hardest, losing weight and becoming anxious, while the males were more aggressive but less anxious.
3. The "Night Owl" Problem: If you are a human who stays up late and tries to exercise at 6:00 AM (your biological night), you might be doing more harm than good, stressing your body out instead of building it up.

In a nutshell: Your body has a schedule. If you try to force it to work hard when it wants to rest, it panics. If you work with its schedule, it thrives. And for females, getting the timing wrong seems to have a much bigger impact than for males. So, listen to your internal clock before you hit the gym!

Technical Summary

1. Problem Statement

While physical exercise is known to provide broad health benefits, emerging evidence suggests that the timing of exercise relative to the circadian cycle significantly influences physiological and behavioral outcomes. However, the specific interplay between circadian phase, sex, and exercise modality remains poorly understood, particularly in preclinical models. Most existing studies focus on running paradigms (treadmill/wheel) or acute stress models (forced swim test for depression), often neglecting the nuanced behavioral responses during repeated, sub-acute exercise sessions. Furthermore, there is a lack of data on how exercise performed during the animal's rest phase versus active phase differentially affects stress markers, body weight, and stereotyped behaviors in both male and female mice.

2. Methodology

The study utilized a rigorous experimental design involving young adult wild-type mice (C57BL/6J, 8–10 weeks old) to isolate the effects of circadian timing and sex.

• Experimental Groups & Light Cycles:

  • Mice were housed in two distinct 12:12 light/dark cycles:
    • Regular (REG): Lights ON at 7:00 AM / OFF at 7:00 PM (Standard lab cycle). Exercise occurred at ZT2–3 (10:00 AM – 12:00 PM), corresponding to the rest phase.
    • Inverted (INV): Lights OFF at 10:00 AM / ON at 10:00 PM. Exercise occurred at ZT2–3 (12:00 PM – 2:00 PM), corresponding to the active phase (darkness).
  • Groups included Male/Female $\times$ Exercise/Sedentary controls.

• Swimming Protocol:

  • Duration: 21 days of daily group swimming.
  • Acclimation: 4-day habituation (5 min to 20 min), followed by 21 days of 20-minute sessions.
  • Conditions: Water at 32°C in a large tank with a divider; soap added to reduce surface tension.
  • Behavioral Quantification: Researchers manually scored six specific behaviors during the last 10 days (focusing on the final 15 minutes of each session) to minimize novelty effects:
    1. Active: Escape attempts, Collisions, Climbing.
    2. Passive: Floating (>50% body above water), Nostrils-Above-Water Events (NAWEs - a novel behavior where the mouse floats vertically with only nostrils exposed), Defecation.
  • Derived Scores: An "Immobility Score" (sum of floating, NAWEs, escape attempts) and an "Active Score" (sum of climbing, collisions).

• Post-Exercise Assessments (24 hours after final session):

  • Open Field Test (OFT): Conducted at the same circadian time as swimming to measure locomotion, anxiety (time in center), rearing (supported/unsupported), and grooming.
  • Physiological Measures: Daily body weight tracking and serum corticosterone (CORT) levels via ELISA.

3. Key Contributions

• Novel Behavioral Ethogram: Identification and quantification of Nostrils-Above-Water Events (NAWEs) as a distinct, stereotyped passive coping behavior during swimming.
• Sex-Circadian Interaction: Demonstration that sex and circadian phase do not act independently but interact to shape exercise responses, with the rest phase (REG) inducing significantly stronger physiological and behavioral stress responses than the active phase.
• Paradigm Shift: Moving beyond the "forced swim test" (depression model) to analyze swimming as a chronic exercise intervention, revealing that exercise during the rest phase acts as a potent physiological stressor.

4. Key Results

A. Body Weight (BW) and Metabolism

• REG Mice (Rest Phase Exercise): Showed significant weight loss or reduced weight gain. Specifically, female mice in REG were the only group to lose significant weight (98.3% of baseline). Male REG mice also gained less weight than sedentary controls.
• INV Mice (Active Phase Exercise): Showed no weight loss; exercised males actually gained more weight than sedentary controls.
• Sedentary Controls: Mice in REG conditions gained more weight than those in INV, suggesting that the daily handling required for weighing in the light phase (disrupting sleep) acted as a mild chronic stressor leading to metabolic changes.

B. Swimming Behaviors

• Active Behaviors: REG mice exhibited significantly more escape attempts than INV mice. Collisions were sex-dependent, with REG males colliding more than REG females; this difference vanished in INV mice.
• Passive Behaviors:
  • Floating: Significantly higher in female REG mice compared to male REG mice. This sex difference was abolished in INV mice.
  • NAWEs: Significantly more prevalent in REG mice (both sexes) compared to INV mice, indicating a shift toward energy-conserving passive coping strategies during the rest phase.
• Coping Scores: The Immobility Score was significantly higher in female REG mice compared to male REG mice, highlighting a sex-specific vulnerability when exercising during the rest phase.

C. Open Field Test (OFT) & Anxiety

• Locomotion: Only female INV mice showed increased total distance traveled compared to sedentary controls, suggesting an anxiolytic or stimulatory effect of active-phase exercise.
• Anxiety (Center Time): Female REG mice spent significantly less time in the center of the arena compared to sedentary controls, indicating an anxiogenic effect of rest-phase exercise.
• Rearing & Grooming: Supported rearing increased in exercised INV mice. Grooming time (a stress indicator) was reduced in exercised INV females but remained unchanged in REG females.

D. Corticosterone (CORT) Levels

• Rest Phase (REG): Exercise caused a massive, significant increase in baseline CORT levels in both male and female REG mice compared to sedentary controls.
• Active Phase (INV): Exercise did not increase CORT levels in INV mice.
• Sex Differences: In INV conditions, females had higher baseline CORT than males regardless of exercise. In REG conditions, exercise elevated CORT in both sexes equally.
• Correlation: Strong positive correlations were found between CORT levels and active behaviors (escapes, collisions) in females, and between passive behaviors (floating, NAWEs) and CORT in a sex- and phase-dependent manner.

5. Significance and Conclusion

• Circadian Timing is Critical: The study provides robust evidence that exercising during the rest phase (light phase for nocturnal rodents) acts as a severe physiological stressor, leading to weight loss, elevated stress hormones (CORT), and anxiety-like behaviors. Conversely, exercising during the active phase (dark phase) yields different behavioral outcomes with lower stress reactivity.
• Sex-Specific Vulnerability: Female mice are particularly vulnerable to the negative effects of rest-phase exercise, showing the most pronounced weight loss, immobility, and anxiety-like behaviors.
• Implications for Preclinical Research: The findings underscore the necessity of explicitly controlling and reporting the circadian phase in preclinical exercise studies. Failure to account for the timing of exercise relative to the animal's light cycle can lead to contradictory results regarding the benefits or harms of exercise.
• Chronotherapeutic Potential: The data suggests that for exercise to be a beneficial intervention rather than a stressor, it should be timed to coincide with the organism's active phase.

In summary, this paper establishes that circadian phase and sex are non-negotiable variables in exercise physiology, fundamentally altering behavioral coping strategies, metabolic outcomes, and stress axis activation in mice.