Sleep Duration And Risk Of Myocardial Infarction
If genetic predisposal to coronary artery disease and individual sleep patterns are accounted for, is there any association between sleep duration and risk of myocardial infarction?
For a long time, scientists thought that our brain and body were inactive during sleep. But studies during the 21st century have proved to differ that thought.
Indeed during our sleep, various processes are going on in our body; including our brains and various organs.
Now let us take a deeper look at sleep. What is sleep what does it consist of?
The sleep cycle is divided into two parts:
- REM (Rapid eye movement)
Sleep begins in the Non-REM cycle; most of our night’s rest is spent here. Initially, it is light N1 and later on progresses to the N3. As we move from N1 to N3 our sleep deepens and we become less responsive to the outside world. About 50 percent of our sleep is spent in the N2 phase, that studies have hinted, maybe be the stage where we file away long term memories.
The rapid eye movement stage is so termed due to the way our eyes dart back and forth behind our eyelids. Dreams mostly happen in this stage. Our sympathetic nervous system is most active during REM.
- 3 possible findings in non-REM sleep; sleepwalking, night terrors, and bedwetting.
- 5 possible findings in REM sleep; dreaming, loss of motor tone, possible memory processing function, erection, and increased brain oxygen use.
- 4 physiological actions found in REM sleep; increased/variable pulse, rapid eye movements, increased/variable blood pressure, and penile/clitoral tumescence
We usually go through the REM and Non-REM cycles several times during the night. Each time the REM stage gets longer whereas the N3 of Non-REM tends to reduce. When we sleep our brain cells have a considerate reduction in firing activity as compared to when we are awake during the day. Our brain starts to fire in a more steady and consistent manner. But during the REM stage of sleep, our brain’s firing is just as when we are awake.
Daghlas I, Dashti HS, Lane J, et al.
Sleep Duration and Myocardial Infarction. J Am Coll Cardiol2019;74:1304-1314.
A study was conducted by UKB (UK bio bank), with all participants free from cardiovascular disease. Participants were divided into short sleep duration- less than 6 hours, or long sleep duration- more than 9 hours and compared to a control group with reported sleep between 6 to 9 hours.
A two-sample Mendelian randomization was conducted for short (24 single nucleotide polymorphisms) and continuous (71 single nucleotide polymorphisms) sleep duration with MI (n = 43,676 cases/128,199 controls), and replicated results in UKB (n = 12,111/325,421).
- Participants with regularly sleeping 7-8 hours were more likely to report excellent health
- Participants who reported a habitually short sleep duration had a 20% higher adjusted risk for incident MI (HR, 1.20; 95% confidence interval [CI], 1.07- 1.33) compared with the reference group (6-9 hours’ sleep duration
- Among participants who reported habitually longer sleep duration, a 34% higher risk for incident MI was observed (HR, 1.34; 95% CI, 1.13-1.58) compared to sleep durations of 6-9 hours.
It can be said that the results obtained support short sleep duration as a potentially causal risk factor for MI.
How can we explain sleep duration being linked to cardiovascular diseases?
To do so, let us take a deeper look one of the processes that occur during sleep.
• The hormone perspective
When we sleep, our body is in the constant process of producing hormones or lowering them, for example, cortisol that is very relatable to stress goes down. When sleep time in not adequate the body produces more cortisol as a stress-response mechanism. High levels of cortisol have been proven to be a high-risk factor to suffer to from cardiovascular disease. Studies have supported this claim that high cortisol levels can increase blood cholesterol levels, triglycerides, blood sugar and blood pressure. The latters being common risk factors for cardiovascular diseases
2. Leptin and ghrelin
These two hormones are the ones that control hunger. With lack of sleep, the balance of leptin and ghrelin is disturbed leading to over-eating and considerable weight gain. Without ample sleeping time, the body’s level of ghrelin is increase whereas that of leptin is reduced leading to an increase in appetite and subsequently obesity over time. Obesity is high risk factor for diabetes and cardiovascular disease.
• The sympathetic nervous system perspective
Short sleep duration patients have been studied and it is agreed that short or inadequate sleeping time causes an increase in firing of the sympathetic nervous system activity. Hence raising the heart rate, causing vasoconstriction as well as salt retention. These factors contribute to hypertension caused by volume overload and an overloaded heart. Hypertensive heart disease includes heart failure, thickening of the heart muscle, coronary artery disease, and other conditions. Cardiac hypertrophy is an independent risk factor for myocardial infarction
As discussed above, it is obvious and proven that myocardial infarction can be linked to inadequate sleep patterns and duration. To lead a healthy life, good sleep should be a must and top priority in every individual’s life