If you smoke, you get less sleep and a “lower quality of rest” than non-smokers do, according to today’s Daily Mail.
The links between smoking and serious, potentially fatal conditions (such as lung cancer and heart disease) are well known. But this headline stems from a recent study suggesting that those of us who enjoy a puff may have sleepless nights too.
The news is based on the results of a German case-control study that recruited smokers and non-smokers from the general population. The researchers asked them to complete a questionnaire on several sleep-related factors, such as how long it took them to fall asleep after going to bed.
The researchers found that current smokers were more likely to report poor sleep quality, even after certain factors that could explain the link were accounted for, such as stress and alcohol consumption.
The researchers also found that, among smokers, more nicotine dependence and smoking intensity were associated with less sleep each night.
They speculated that there could be a direct biological cause and effect relationship between cigarette smoking and poor sleep, possibly due to the stimulant effects of cigarette ingredients, such as nicotine.
But the current study can't prove this theory, as there are other possible explanations for the findings, including other lifestyle choices not accounted for (such as a poor diet, which has previously been linked to poor sleep). Furthermore, it is not clear whether poor sleep was likely to increase the amount the participants smoked, or whether smoking decreased their sleep quality, or both.
What the study does suggest is that improved sleep may be just one of a range of important health benefits that come if you stop smoking.
Where did the story come from?
The study was carried out by researchers from different research centres and universities in Germany. It was funded by the German Research Foundation.
The study was published in the peer-reviewed journal Addiction Biology.
The Daily Mail reported the results of this study accurately.
What kind of research was this?
This was a case-control study that recruited smokers and non-smokers randomly from the general population. It aimed to determine whether smoking has an effect on sleep quality in people without a history of psychiatric disorders, as this may skew any association seen.
It then aimed to determine whether, among smokers, any of the following factors have an influence on the different aspects of sleep quality:
- the degree of nicotine dependence
- the number of cigarettes smoked each day
- smoking urges
- blood cotinine levels
- exhaled carbon monoxide (CO) (markers for exposure to tobacco smoke)
This was a well-designed trial, but due to the study design it can’t show whether smoking causes sleep disturbances. This is because there are other factors that could explain any link seen. For example, smokers could watch more TV or have a poorer diet, both of which are thought to affect sleep.
Furthermore, we don’t know whether poor sleep increases the amounts people smoke, or whether smoking decreases sleep quality (this type of uncertainty is known as a temporal bias, which is where we are unsure whether “A leads to B” or vice versa).
A randomised controlled trial could perhaps test whether smoking is responsible for poor sleep quality, although this is highly unlikely to be performed for ethical reasons due to the other, more serious harms associated with smoking.
What did the research involve?
The researchers recruited 1,071 current smokers (defined as smoking at least seven cigarettes a week) and 1,243 people who had never smoked (or who only had smoked a maximum of 100 cigarettes in their lifetime), from the general population in Germany. To be included in the study people had to be aged between 18 and 65 years old, and to be free from alcohol, substance use and psychiatric disorders. The researchers evaluated sleep quality in smokers and non-smokers by asking them to complete a questionnaire that collected data on:
- sleep quality
- sleep latency (the amount of time it takes to fall to sleep)
- sleep duration
- habitual sleep efficiency (the proportion of time actually spent asleep while in bed)
- sleep disturbances
- use of sleep medication
- daytime dysfunction
The researchers also collected data on the level of smoking and the level of nicotine dependence that the smokers experienced. To determine the level of nicotine dependence, the smokers completed another questionnaire: the Fagerström Test of Nicotine Dependence. The smokers were also asked how many cigarettes they smoked daily and for how long they had smoked (to determine their lifetime consumption).
Plasma levels of cotinine (a substance that is formed when nicotine is broken down inside the body) and levels of exhaled carbon monoxide (CO) were measured.
Smokers were also asked to complete the Questionnaire of Smoking Urges, which is designed to assess how frequently people experience cravings for cigarettes.
The researchers looked to see if there was an association between smoking and sleep disturbance, and whether this association was present after they had adjusted for a number of variables that may explain the differences (confounders), including:
- body mass index (BMI)
- level of education
- depression symptoms
- attention deficit hyperactivity disorder (ADHD) symptoms
- alcohol consumption
- perceived stress
The researchers also investigated whether the level of nicotine dependence, the number of cigarettes smoked per day, smoking urges, blood cotinine levels and exhaled CO have an influence on sleep quality and duration.
What were the basic results?
Significantly more smokers (28.1%) than non-smokers (19.1%) demonstrated overall poor sleep quality. After adjusting for potential confounders, the smokers demonstrated increased risk of disturbances on the following sleep measures:
- Sleep latency: smokers required increased time needed to get to sleep (odds ratio [OR] 1.42).
- Sleep duration: smokers had reduced amount of sleep per night (OR 1.98).
- Global sleep quality: smokers had reduced sleep quality (OR 1.35).
However, smokers were at reduced risk of daytime dysfunction (known as daytime sleepiness) (OR 0.66). The researchers speculated that this may be due to the “activating properties of nicotine”.
The researchers also found that among smokers, higher degrees of nicotine dependence and intensity of smoking were associated with less sleep each night.
- Levels of exhaled CO, number of cigarettes reportedly consumed each day and plasma cotinine levels were all associated with reduced sleep duration.
- A high degree of nicotine dependence, high intensity of smoking urges and number of cigarettes smoked each day were associated with increased sleep latency.
- Heavily dependent smokers suffered more sleep disturbances.
- The number of cigarettes smoked each day was associated with poor overall sleep quality.
How did the researchers interpret the results?
The researchers concluded that this study demonstrates “an elevated prevalence of sleep disturbance in smokers compared with non-smokers in a population without lifetime history of psychiatric disorders, even after controlling for potentially relevant risk factors”.
This case-control study has shown that current smokers are more likely to report poor sleep quality. This was a well-designed trial but with some limitations:
- Due to the study's design, it can't show whether smoking causes sleep disturbances. This is because there are other factors that could explain any link seen. For example, smokers could make other unhealthier choices, such as watching more TV or drinking more coffee than non-smokers do.
- As the researchers didn't follow people up over time, we don’t know which came first: whether poor sleep increased the amount the participants smoked, or whether their smoking decreased their sleep quality, or a bit of both.
- The complex interplay of factors, including some the researchers measured such as alcohol consumption, means that this study is a first step into untangling several behaviours and their underlying causes, but at this stage seems exploratory.
Alongside all the well-proven reasons to stop smoking, improved sleeping may not be high on the list, but insomniacs who smoke could be well advised to try stopping.