Nighttime Awakenings Harm Sleep-Deprived Hearts
We’ve all experienced a situation where a particularly good dream or night of deep sleep is interrupted by an alarming noise, urge to go to the bathroom, or sometimes for no apparent reason at all. These nightly awakenings, technically termed “spontaneous nocturnal arousals”, may seem relatively harmless. Most people have a fine time going right back to sleep following a nocturnal arousal, and do so fairly quickly.
However, the cardiovascular impact of these nighttime arousals may actually be significant from a long-term health perspective. Several things happen when one awakes from sleep, including a brief and transient “spike” in heart rate and blood pressure, which may also be accompanied by an increase in autonomic sympathetic nervous system activity. Since these cardiovascular stimulations are relatively short-lived, they may not pose a substantial risk to health. However, an augmented cardiovascular response to an awakening or a response that takes longer to subside (i.e. return to baseline) could be harmful for the heart and blood vessels.
As important as sleep quality is (sleep quality being also related to nocturnal arousals or awakenings), sleep quantity (how much sleep you get per night) has been more strongly associated with increased cardiovascular disease risk, among others. What is less known is how short sleep duration elevates cardiovascular risk in sleep-deprived individuals.
As a new study published in the American Journal of Physiology: Heart and Circulatory Physiology suggests, habitual short sleep duration might augment the cardiovascular response to nighttime awakening and therefore, negatively impact acute cardiovascular health and chronic cardiovascular disease risk. This could be one link between a reduced nightly sleep duration and cardiovascular diseases.
Brief study methods
Healthy adult men (20) and women (15) were recruited for this study. For about 11 days on average, all participants had their sleep monitored at home using wrist-worn activity monitors. These data were then used to categorize participants as short sleepers or normal sleepers; receiving <6.93 or >6.93 hours of sleep per night, respectively. This cutoff point for short or normal sleep falls right in line with current sleep recommendations that advocate for adults to receive >7 hours of sleep each night for optimal health.
In addition to baseline habitual sleep characteristics, participants underwent a single overnight laboratory polysomnography sleep assessment which measured brain wave activity (EEG), electrocardiography (ECG), electromyography (EMG), airflow, blood oxygen levels, and limb movements throughout one entire sleep period.
The primary outcome of this study was the heart rate response to spontaneous arousals — how much did heart rate increase during each awakening and how long did it take for heart rate to return to baseline (heart rate recovery). The heart rate response was then compared between the short sleepers and normal sleepers to determine the impact of habitual sleep duration on this variable.
Results
One “result” to highlight first is that there was no baseline difference in sleep characteristics between the short sleep and normal sleep groups other than their habitual sleep duration. This is important because it allows the findings to be directly related to sleep duration, and not some other variable such as sleep quantity or an uneven distribution of sleep disorders between these groups.
Given that interlude, let’s take a look at some of the main findings.
- Heart rate during recovery from nighttime arousals was significantly elevated in the short sleep group vs. the normal sleep group
In the figure below, you can observe that beginning at cardiac cycle number 10 after the arousal onset, heart rate in the short sleep (SS) group was elevated significantly compared to the normal sleep (NS) group until cardiac cycle 24.
- Contrary to the initial hypothesis of the authors, there were no differences in the heart rate response to arousal between men and women
Interestingly, this finding is a bit in contrast to what some literature has shown — a heightened (i.e. more adverse) cardiovascular response to sleep deprivation among women compared to men. As you can see in the graph below, there were no sex differences in heart rate reactivity to the nighttime arousals in this study.
While it may seem like this is just another study telling us that we should sleep more, I think that the implications are a bit more interesting than that. Clearly it is important to get enough sleep, given the known impact that insufficient sleep can have on daytime metabolic, cognitive, and physical function. These data provide compelling evidence that habitually falling short on sleep can exert negative effects during the night as well.
The night is a time when our body — and in particular our nervous system and cardiovascular system — goes into a restoration mode. Sleep is essential to give these systems a break. If you are routinely missing out on the 7–9 hours recommended for most adults, it seems like you might be putting your cardiovascular and nervous systems at risk by reactivating them during the night. Over time, this could have effects ranging from cardiovascular remodeling to elevated blood pressure and eventually, the development of one of many cardiovascular diseases.
Show your heart some love, and give it at least 7 hours of rest each night and, ideally, at the same time each night. A consistent, regular sleep habit is one of the best things you can do for your cardiovascular health, and the benefits extend well beyond that.
Study cited
