Circadian rhythms are biological cycles that occur in humans, animals, insects, plants, and even bacteria with a period of approximately (circa) one day (diem). These rhythms are determined internally by a part of our hypothalamus and are synchronized perfectly to our 24-hr days by the sun and other cues. This internal clock mediates daily variation in everything from hormone levels, to sleep/wake cycles, feeding behaviour, thermoregulation, to bowel movements and cardiovascular function, among many others.
It is largely due to these predictable circadian rhythms that risk of a myocardial infarction (heart attack) is significantly highest in the morning (by about 40% as compared to other times in the day). Right as we awake, our cardiovascular system is in the most compromised state –systolic blood pressure and heart rate show the largest upward spike in the morning, blood vessels ability to dilate in response to increased blood flow is compromised (relative endothelial dysfunction), blood clots are more likely to form, and the ability to break them up is at its lowest point in the day. Is it any wonder then, that the first snowfall – shoveled early in the morning by people at risk – always leads to a spike in heart attacks?
Interestingly, the 1hr shift experienced by citizens of many countries (most notably Europe and North America) during the fall and spring in accordance with daylight savings time also has a detrimental effect on cardiovascular risk. The problem lies in the fact that our circadian clock takes time to adjust, and it is best adjusted by changes in day/night or light/dark cycles – not simply the adjustment of our watch. Thus, the few mornings after the clock change our internal clocks are at odds with our watches and particularly in the spring – when one hour of sleep is lost, we wake up with our cardiovascular system being in an even more compromised state than normal.
A 2009 study in the New England Journal of Medicine clearly shows this effect. In the study the authors investigated the number of heart attacks in Sweden the week before and the week after the 1hr clock changes in both the spring and fall. As would be predicted, individuals had an approximately 5% greater risk of having a heart attack immediately after the ‘spring ahead’ clock change compared to the previous week.
The authors rightfully suggest that individuals at risk of cardiovascular complications would be better off changing their clocks more gradually (i.e. by 15 minutes, starting on the Friday before the change). More importantly, avoiding strenuous activity and stress right in the morning may also be a valid suggestion.
An even better strategy from a public health standpoint would be to do away with daylight savings time altogether.