Understand the Relationship Between Sleep Disorders and Type 2 Diabetes

Dr Kevin Fernando Scrutinises the Latest Evidence on the Impact of Poor Sleep Quality on Cardiometabolic Health, Offering Practical Advice for Primary Care Practitioners

For an activity that takes up around one-third of our lives, sleep receives much less attention in healthcare than it deserves. Sleep is positioned in the foundation of Maslow’s Hierarchy of Needs, alongside food, shelter, air, and water (see Figure 1),¹ and even though the universality of Maslow’s theory is an issue of debate,^2,3 it is clear that we need sleep to function at the most basic level. Hence, sleep disorders, which are an underappreciated and pervasive public health problem, have wide-ranging effects on health.

Figure 1: Maslow's Hierarchy of Needs

A recent online survey found that there was a considerable decline in sleep quality among adults in the UK between 2021 and 2022.⁴ Overall, 74% of adults who responded to the survey reported experiencing lower-quality sleep in 2022 than in the preceding year, with younger adults aged 35–44 years getting the least sleep—nearly 50% of this group slept for just 5–6 hours per night.⁴ Worryingly, around 10% of respondents were only getting 2–4 hours of sleep per night, significantly fewer than the recommended 7–8 hours.⁴

The COVID-19 pandemic has contributed to this decline in sleep quality;⁵ however, rates of both suboptimal sleep duration and poor-quality sleep have been high for many years,^6,7 and are inextricably linked to our modern, 24-hour lifestyles.⁸ We can now binge-watch television and films, order online shopping, and get fast food delivered 24 hours a day, and psychological stressors, alcohol consumption, lack of exercise, use of electronic devices, and exposure to blue light all contribute to disordered sleeping.⁸ Significantly, less than 2% of hunter-gatherer populations in Namibia and Bolivia show signs of insomnia, compared with up to 30% in industrial societies.⁹

Moreover, the current epidemics of type 2 diabetes and obesity are fundamentally linked to this decline in sleep quality: a higher prevalence of obstructive sleep apnoea (OSA) and obstructive sleep apnoea/hypopnoea syndrome (OSAHS) is associated with both conditions,^10,11 and nocturnal symptoms of hypoglycaemia, urinary frequency, and painful peripheral neuropathy are all associated with type 2 diabetes,^12,13 particularly a longer-standing diagnosis.

In this article, I will discuss the relationship between type 2 diabetes and sleep, providing information and practical tips for primary care clinicians on reducing the long-term impacts of poor sleep quality and quantity and sleep disorders on cardiometabolic health.

The U-Shaped Association Between Sleep Duration and Risk of Type 2 Diabetes

In 2015, a high-quality meta-analysis of prospective studies involving nearly 500,000 participants was published that demonstrated a U-shaped association between sleep duration and risk of type 2 diabetes.¹⁴ Both excessively short and excessively long sleep durations were found to be associated with elevated risk of type 2 diabetes.¹⁴ The sleep duration associated with the lowest risk of type 2 diabetes was determined to be between 7 and 8 hours.¹⁴ Indeed, when compared with 7 hours per night, a reduction or increase of 1 hour of sleep was associated with a 9% or 14% increased risk of developing type 2 diabetes, respectively.¹⁴

Furthermore, a recent historical cohort study found that people with insomnia had a 16% higher risk of developing type 2 diabetes than people without insomnia, and that this risk was almost double in people with insomnia aged less than 40 years.¹⁵ Duration of insomnia was also a contributing factor: people with a duration of insomnia of less than 4 years had a 14% higher risk of developing type 2 diabetes, whereas those whose duration of insomnia was more than 8 years had a staggering 51% increased risk.¹⁵

How Can We Explain the Link Between Sleep Duration and Risk of Type 2 Diabetes?

Although definitive evidence is lacking, there are several plausible underlying mechanisms that may explain the link between short sleep duration and risk of type 2 diabetes. Various neuroendocrine changes have been observed during short sleep duration, including changes in cortisol, glucocorticoid, and growth hormone levels,¹⁶ all of which are known to drive insulin resistance.¹⁷ Short sleep duration is also associated with increases in hunger, appetite, and energy consumption,¹⁸ which may lead to overweight or obesity and, in turn, insulin resistance.¹⁹ In addition, mechanistic studies have shown worsening systemic inflammation and changes in leptin and ghrelin levels (hormones that affect appetite) as a result of short sleep duration, which have a negative impact on cardiometabolic health.^18,20 There are multiple other mechanisms that may drive this association between sleep duration and risk of type 2 diabetes.¹⁴

The association between too much sleep and risk of type 2 diabetes is more difficult to explain. Depression, low socioeconomic status, low levels of physical activity, and sleep disorders (including OSAHS and restless legs syndrome) are all associated with excessive sleep^14,21 and are themselves risk factors for type 2 diabetes, so may be responsible for the increased risk of adverse cardiometabolic disease.

Regardless of the underlying mechanisms, it may be beneficial for clinicians to give general advice about improving sleep duration to patients who sleep too much or too little, and to inform them of the associated increased risk of type 2 diabetes. This advice may help to reduce the prevalence of the condition.

Short Sleep Duration and Risk of Diabetes Progression

Short sleep duration is not only associated with an elevated risk of type 2 diabetes: it is also linked with an increased risk of progression from prediabetes to type 2 diabetes. A 2022 systematic review and meta-analysis found that short sleep duration (less than 5–6.5 hours per night) was associated with a 59% increased risk of progression from prediabetes to type 2 diabetes.²² Accordingly—alongside the usual signposting, lifestyle advice, and referral to the NHS Diabetes Prevention Programme²³ where appropriate—primary care practitioners should counsel their at-risk patients about the importance of adequate sleep duration to mitigate progression from prediabetes to type 2 diabetes.

The Impact of Nightshift Work

The authors of this systematic review and meta-analysis also highlighted the negative association between nightshift work and risk of diabetes progression.²² These findings mirror those of previous research, in which short sleep duration and circadian misalignment—particularly relating to nightshifts—were linked with reduced insulin sensitivity and increased risk of developing type 2 diabetes.²⁴ Patients who work nightshifts may benefit from knowing about this association, particularly if they have already been identified as at risk.

How Do Quality and Duration of Sleep Impact Established Type 2 Diabetes?

Beyond risk of type 2 diabetes, there is strong evidence linking sleep quality and duration with clinical markers associated with diabetes, including glycaemic regulation,^25,26 glucose tolerance,²⁶ insulin sensitivity,^25,26 blood pressure,²⁷ glycated haemoglobin (HbA_1c),^26,28 and lipid profiles.²⁹

Association Between Sleep Duration and HbA_1c Level

A systematic review and meta-analysis from 2017 identified that overly long (more than 8 hours) and overly short (less than 6 hours) sleep durations are linked to poorer health outcomes, particularly higher HbA_1c level.^28,30 The study also found that poor quality sleep was associated with an HbA_1c level 0.35% higher, on average, than normal sleep.²⁸ If the associated HbA_1c improvement of normal sleep is extrapolated to the results of the seminal UK Protective Diabetes Study,^28,30 this equates to a clinically significant 3% reduction in death from type 2 diabetes, a 2% reduction in myocardial infarction, and a 5% reduction in microvascular disease.³¹

The authors of the systematic review concluded that sleep health is an important and modifiable risk factor for improving glycaemic control in people living with type 2 diabetes.²⁸ Therefore, counselling about sleep should form an integral part of any diabetes intervention.

Effect of Irregular Sleep and Chronotype

Irregular sleep is also associated with a suboptimal glucose level in patients with type 2 diabetes;^12,32 conversely, sleep regularity is often affected by the increased prevalence of sleep disorders in type 2 diabetes, including insomnia, OSAHS, and restless legs syndrome.³⁰ A person’s chronotype (their natural inclination to sleep at a certain time, for example, ‘early birds’ or ‘night owls’) also plays a pivotal role, as people with evening chronotypes may be more susceptible to inactivity and a suboptimal glucose level.³⁰

ADA/EASD Emphasis on Sleep for Improving Cardiometabolic Health

In light of this, the recently updated American Diabetes Association (ADA)/European Association for the Study of Diabetes (EASD) consensus report positions sleep as a key 24-hour physical behaviour for managing type 2 diabetes, alongside sweating, sitting, strengthening, and stepping.³⁰ The effects that these behaviours have on different indicators of cardiometabolic health are outlined in the ADA/EASD consensus report.³⁰

People living with type 2 diabetes should aim for consistent, uninterrupted sleep, both in the week and at the weekend. There needs to be more awareness among healthcare practitioners and the public of the long-term effects of poor sleep on cardiometabolic conditions, including type 2 diabetes, hypertension, and cardiovascular disease, and how these effects can be mitigated.

Type 2 Diabetes and Obstructive Sleep Apnoea: A Bidirectional Relationship

Obstructive Sleep Apnoea/Hypopnoea Syndrome

NICE Guideline 202, Obstructive sleep apnoea/hypopnoea syndrome and obesity hypoventilation syndrome in over 16s,¹⁰ emphasises that OSAHS is a common and under-recognised condition. In fact, OSAHS is estimated to affect around 6% of adult women and 13% of adult men in the UK,³³ yet a considerable proportion of these people remain undiagnosed.¹⁰

If untreated, OSAHS is associated with neurocognitive impairment, cardiometabolic disease, and all-cause death.³⁴ Furthermore, individuals living with OSA are up to 4.9 times more likely to have a road traffic accident than those without,³⁵ and 10 times more likely if living with undiagnosed OSA,³³ so this is a diagnosis that affects both the person living with the condition and the general public. For information regarding the Driver & Vehicle Licensing Agency ’s requirements for driving with OSA, visit bit.ly/3Yzwza7.

How are OSAHS and Type 2 Diabetes Related?

OSAHS is more prevalent in people living with type 2 diabetes than in the general population.³⁶ Conversely, around 15–30% of people living with OSAHS have type 2 diabetes.³⁶

OSAHS is also more prevalent in people living with certain other conditions, including:³⁷

• obesity or overweight
• cardiac arrhythmia (particularly atrial fibrillation)
• transient ischaemic attack or stroke
• chronic heart failure
• moderate or severe asthma
• polycystic ovary syndrome
• hypothyroidism
• treatment-resistant hypertension.

A recent cohort study attempted to disentangle the complex relationship between OSA and type 2 diabetes, identifying that OSA is associated with a 37% increased risk of type 2 diabetes (independent of demographic, lifestyle, comorbidities, and anthropometric factors), and that type 2 diabetes treated with insulin therapy is associated with a 43% higher risk of OSA (independent of adiposity).¹¹ The authors concluded that increased clinical awareness of this bidirectional relationship may improve prevention and treatment of both type 2 diabetes and OSA.¹¹

Actions in Primary Care

It may be prudent for primary care clinicians to act upon this bidirectional relationship to identify patients living with these comorbidities. Clinicians can consider measuring HbA_1c level in all patients diagnosed with OSAHS not known to have type 2 diabetes, and conversely screening for OSAHS in all patients living with type 2 diabetes not known to have any sleep disorders—the Epworth Sleepiness Scale (epworthsleepinessscale.com)³⁸ is the standard screening option, and the STOP-Bang Questionnaire (www.stopbang.ca/osa/screening.php)³⁹ can also be considered.¹⁰

OSA, Maculopathy, and Retinopathy

OSA is known to be involved in the pathogenesis of maculopathy and retinopathy in patients with type 2 diabetes. A 2017 longitudinal study found that OSA (even when mild) was independently associated with maculopathy and sight-threatening diabetic retinopathy (DR) in people living with type 2 diabetes.⁴⁰ OSA was also an independent predictor of progression to preproliferative and proliferative DR over 4 years.⁴⁰ Importantly, patients treated with continuous positive airway pressure (CPAP) therapy were less likely to progress in this way.⁴⁰

With this in mind, practices may wish to identify patients with mild or moderate nonproliferative DR and screen them for OSAHS using the Epworth Sleepiness Scale³⁸ and STOP-Bang Questionnaire.³⁹ Early diagnosis and treatment of underlying OSAHS, particularly with CPAP, have the potential to reduce progression of DR and prevent blindness.⁴⁰

The Basic Tenets of Healthy Sleep Hygiene for all Patients

Box 1 compiles typical sleep hygiene advice for all patients, not just people living with type 2 diabetes, and Box 2 contains useful resources relating to sleep hygiene that may be useful to patients and clinicians alike. As always, clinicians should individualise, modify, and compromise on their recommendations where necessary.

Conclusion

As Leonardo da Vinci said, ‘a well-spent day brings happy sleep’,⁴⁵ and sleep is certainly an underappreciated aspect of good health that is important for all patients. If I were to give three key take-home messages for primary care clinicians on this topic, they would be that:

• primary care needs to wake up to sleep as a major metabolic risk factor—clinicians should enquire about and counsel patients on sleep quality and duration in every diabetes consultation
• OSAHS is a sleeping giant—this condition is common in those living with type 2 diabetes, but remains underdiagnosed, and is associated with cardiometabolic disease and all-cause death; screen for OSAHS in all people living with type 2 diabetes who are not known to have any sleep disorders
• sleep hygiene is more than just a bedtime buzzword—advise patients on how to practise healthy sleep hygiene, but individualise, modify, and compromise on this advice where necessary.