Question

Social jetlag refers to the difference between circadian and social clocks, and is measured as the difference in sleep and wake times between work days and free days. For example, if you sleep between \(11 \mathrm{pm}\) and 7 am on weekdays but from 2 am to 10 am on weekends, then your social jetlag is three hours, or equivalent to flying from the West Coast of the US to the East every Friday and back every Sunday. Numerous studies have shown that social jetlag is detrimental to health. One recent study \(^{64}\) measured the self-reported social jetlag of 145 healthy participants, and found that increased social jetlag was associated with a higher BMI (body mass index), higher cortisol (stress hormone) levels, higher scores on a depression scale, fewer hours of sleep during the week, less physical activity, and a higher resting heart rate. (a) Indicate whether social jetlag has a positive or negative correlation with each variable listed: BMI, cortisol level, depression score, weekday hours of sleep, physical activity, heart rate. (b) Can we conclude that social jetlag causes the adverse effects described in the study?

Short answer

The study determines that social jetlag has a positive correlation with BMI, cortisol levels, depression scores, and heart rate. It has a negative correlation with weekday hours of sleep and physical activity. However, while there is a correlation, it does not necessarily imply causation, and so we can't firmly conclude that social jetlag causes these adverse effects.

Step-by-step solution

Understanding Correlations

For each listed variable, the correlation is described following the results from the study. If social jetlag is increasing alongside the variable, the correlation is positive. If the variable decreases as social jetlag increases, the correlation is negative. To be clear, positive correlation means both factors increase or decrease together, while negative correlation implies that one factor increases while the other decreases.

Identifying the Correlation

From the study, we can identify correlations respectively for each variable: 1. BMI: Positive correlation, as social jetlag increases, BMI is also higher.2. Cortisol levels: Positive correlation, as higher cortisol levels correspond with increased social jetlag.3. Depression score: Positive correlation, as depression score increases with higher levels of social jetlag.4. Weekday hours of sleep: Negative correlation, as the study indicates fewer sleep hours during the week with more social jetlag.5. Physical activity: Negative correlation, there's less physical activity with more social jetlag indicated.6. Heart rate: Positive correlation, as a higher resting heart rate is associated with greater social jetlag.

Understanding Causation

Causation refers to 'cause and effect' when one variable produces an effect or change in another variable. However, just because there is a correlation between two variables, it does not mean that one causes the other. This is a common mistake and is summarized in the phrase 'correlation does not imply causation'.

Concluding Causation

Given the study's findings, social jetlag has positive or negative correlations with different health variables. Yet, inferring causation from these findings is not accurate. While there might be a relationship between the variables, it does not definitively mean that social jetlag is the 'cause' of these effects. The increase in BMI, cortisol levels, and depression scores could be influenced by many different factors. Therefore, based on the information given in the study, it's not conclusive that social jetlag 'causes' the adverse health effects. More rigorous experimental designs, such as randomized controlled studies, are required to determine causality.

Key concepts

Correlation vs Causation

When studying statistics or research findings, it's vital to differentiate between correlation and causation. Correlation indicates a relationship between two variables, where changes in one variable mirror the changes in another. However, it cannot be immediately assumed that one variable is the cause of the change in the other; this is known as causation.

For example, in studies investigating social jetlag, researchers may find that individuals with greater social jetlag also report higher body mass indexes (BMIs) and cortisol levels. This would be described as a positive correlation. On the other hand, a negative correlation might be observed between social jetlag and weekly physical activity — as one increases, the other decreases.

But can we leap from correlation to causation? Not without more evidence. Many factors contribute to health outcomes, and without controlled experiments to isolate variables, it's impossible to claim definitively that social jetlag causes higher BMIs or increased depression. Causal statements need backing by rigorous research methods, such as randomized controlled trials, that can control for outside influences and provide more reliable evidence of cause-and-effect relationships.

Circadian Rhythms

Circadian rhythms are natural, internal processes that regulate the sleep-wake cycle and repeat roughly every 24 hours. These rhythms are influenced by external cues like light and temperature but are driven internally by biological clocks comprised of specific molecules that interact within cells throughout the body.

Your circadian rhythm plays a critical role in determining your ideal timing for restorative sleep. Disruption to these rhythms can lead to a state known as social jetlag, where there's a misalignment between a person's biological clock and their social obligations. This misalignment can create a scenario where individuals may be forced to wake up for work or school when their bodies are still in sleep mode, or to stay awake when their rhythms are signaling that it's time for sleep.

Understanding circadian rhythms is crucial, as they influence not only sleep but also many other bodily functions, including hormone release, eating habits and digestion, and body temperature. When these rhythms are disrupted, as with social jetlag, it can lead to suboptimal functioning, affecting overall well-being and health.

Health Effects of Social Jetlag

Social jetlag is not merely about feeling groggy on a Monday morning; its effects can ripple through various aspects of health. The alignment between our social and biological clocks is often disturbed by our modern lifestyles, leading to noticeable consequences.

Listed below are some of the health effects associated with social jetlag, as indicated by research:

• Weight Gain: Positive correlations have been found between social jetlag and higher BMI, suggesting those with greater social jetlag might experience weight gain more readily.
• Stress: Increased levels of cortisol, a stress-related hormone, have been linked to social jetlag. Chronic elevation of cortisol can lead to numerous health problems.
• Mental Health: Higher scores on depression scales suggest that disrupted sleep and social rhythms could negatively impact mental health.
• Sleep Deprivation: With less sleep during weekdays, one may face the effects of sleep deprivation, including poor cognitive function and mood swings.
• Physical Inactivity: A reduction in physical activity might occur due to the misalignment of biological and social clocks, potentially contributing to a sedentary lifestyle.
• Cardiovascular Strain: A rise in resting heart rate is another potential consequence, which can be a strain on the cardiovascular system over time.

However, while studies may show these associations with social jetlag, these outcomes cannot be solely attributed to it as the direct cause. Health and well-being are multifaceted, and there is a complex interplay of genetic, environmental, and behavioral factors influencing each of these health variables. It's essential to consider the broader context and not to focus on one element like social jetlag in isolation.