The Sleep Index compares and analyses the conditions for sleeping quality in 75 cities around the world. The cities were chosen for being some of the most well-known global metropolises, as well as for their availability of extensive and comparable data. The study consists of nine factors that contribute to the overall quality of a person's sleep:
- Mental Wellbeing (Score)
- Physical Health (Score)
- Caffeine, Nicotine and Alcohol Consumption (Score)
- Overwork + Commute (Score)
- Employment and Finances (Score)
- Chronic Pain (Score)
- Environment (Score)
- Sleep Duration (Minutes)
- Undersleep (%)
The methods used to find each factor are described in detail below. All of the information collected is based on the latest data available.
In cases where a factor consisted of one or more indicators, these were scored and averaged. The equation for scores is as follows:
z-Score = x - mean(X)Standard deviation(X)in short x - μσ
For columns where a low value is better, for example, a low amount of Caffeine, Nicotine and Alcohol Consumption, the score is inverted such that it is attributed a higher score:
z-Scoreinverted = -1*x - mean(X)Standard deviation(X) in short -1 *x - μσ
Where present for a factor, scores are normalised such that 50 equals the lowest value in the final dataset and 100 the highest value in the final dataset. Therefore, the higher the score, the better the city ranks for that factor in comparison to the other cities in the index. The equation for normalization is as follows:
score = (100-50) *x - min(X)max(X) - min(X)+50
Mental Wellbeing Score
The combined rates of mental health disorders and societal stress in a city. The higher the score, the better the mental wellbeing of residents. Mental health and stress are recognised to have significant impacts on sleep quality and can lead to sleep deprivation.
Chronic sleep problems have been reported as high as four times more prevalent in people with mental health disorders than the general population, and are more prevalent in people living with conditions like anxiety, depression and bipolar disorder.¹ Prevalence of depression among the general population. Country-level data. Source: World Health Organisation (WHO) - Global Health Observatory (GHO), 2016.
DALYs (Disability Adjusted Life Years) from anxiety and depressive disorders. Country-level data, with US cities using state-level data. Source: Institute for Health Metrics and Evaluation (IHME) - Global Burden of Disease Study, 2017.
The emergence of the Covid-19 pandemic has resulted in heightened concerns about public and personal health, as well as measures implemented to control the spread of the virus.
COVID-related stress: Country-level survey data on anxiety and psychological stress related to the spread of the pandemic and the conditions that have arisen as a result of the spread. Source: COVIDiStress Global Survey. Results as of August, 2020.
Divorce rates can be seen as a measure of societal stress resulting from the breakdown of the family unit.
Divorce rate: Crude divorce rates at a regional level. Source: Sub-national and national vital statistics departments, latest available data.
Physical Health Score
The combined rates of obesity, physical inactivity and the number of gyms available in a city. The higher the score, the better the physical health of residents. Research has found that physical activity positively impacts sleep quality in young and older groups,² while other studies have found that obesity is associated with an increased prevalence of sleep apnea, potentially causing sleeping problems that reduce sleep quality and duration.³
A lack of adequate sleep has also been suggested to be a contributing factor to a growth in obesity, with most studies finding an association in inadequate sleep and weight gain among children.⁴
Obesity rates: Crude adult obesity rates at national level, US cities use state-level data. Source: World Health Organisation – Global Health Observatory, 2016; Robert Wood Johnson Foundation - State of Childhood Obesity report (adult rates), 2019.
Physical inactivity rates: Prevalence of adult inactivity at national level, US cities use state-level data. Source: World Health Organisation – Global Health Observatory, 2016; Center for Disease Control – Physical Activity database, 2018.
Number of gyms: Quantity of gyms and fitness centres in a city as a proportion of the city population. Search terms ‘leisure=fitness_centre’ and ‘leisure=sports_centre’. Source: OSM Overpass API, data as of August, 2020.
Caffeine, Nicotine and Alcohol Consumption Score
The combined indexes for the intakes of caffeine, nicotine and alcohol in a city. The higher the score, the less stimulants consumed by residents. Certain dietary intake is known to adversely impact sleep quality and duration. These include excess levels of caffeine⁵, nicotine⁶ and alcohol⁷ consumption – especially in the leadup before bed.
Caffeine consumption: Country-level kg per capita consumption of coffee, as well as kg per capita consumption of tea. Source: International Coffee Organization – Trade Statistics database, 2019; US Food and Agriculture Organization – FAOSTAT, 2019.
Nicotine consumption: Smoking prevalence rates and cigarette consumption per capita. Data used is at a country level, US cities use state-level data. Source: World Health Organisation – Global Health Observatory, 2016; Center for Disease Control –State Tobacco Activities Tracking and Evaluation (STATE) System, 2018.
Alcohol consumption: Alcohol consumption in litres per capita. Data used is at a country level, US cities use state-level data. Source: World Health Organisation – Global Health Observatory, 2016; National Institute on Alcohol Abuse and Alcoholism – Surveillance Report, 2018.
Overwork + Commute Score
The percentage of the population working over 48 hours combined with the average commuting time in a city. The lower the score, the more hours a city puts into work and commute.
Percent of the population working over 48 hours: Country level data, US cities use state-level data. Source: International Labour Organisation – ILOSTAT, 2019; Bureau of Labor Statistics, 2018. Commuting time: City level average minutes commuting one-way. Source: US Census – American Fact Finder, 2019; INRIX – Global Traffic Scorecard, 2019; Numbeo, 2020; Various media sources.
Employment and Finances Score
The combined unemployment rate and monthly living costs in a city. The higher the score, the less financial and career stress experienced by residents. Financial and career stress is a factor which contributes to heightened stress and, by extension, a loss of sleep, with research showing that money worries correlate with sleep problems in older people.⁸
Unemployment rate: Harmonised unemployment figures for June, 2020. Metropolitan crude rates taken where available, otherwise regional data was used. In rare cases national data was used. Source: vital statistics departments, latest available data; International Monetary Fund - World Economic Outlook, 2020.
Affordability: Monthly living costs as a share of household income, after tax. A basket of estimated monthly costs includes: basic utilities costs, groceries, internet connection, leisure activities, clothes, and eating out. Source: OECD – Employment Database, 2018; Numbeo – Cost of Living Index, 2020.
Chronic Pain Score
The impact of chronic pain on the health of inhabitants. The higher the score, the less chronic pain experienced by residents. Chronic pain is understood to have a cyclic relationship with sleep: chronic pain is a major contributor to loss of sleep, and a lack of proper sleep produces fatigue and increases sensitivity to pain.⁹ Global disease studies have reported high levels of chronic pain burden in developed countries (although not exclusively).
Research points to a connection between ageing populations (Italy, Germany and Denmark, for example) and an increase in the burden of non-communicable diseases, as these are understood to increase rapidly with age.¹⁰ It is possible that due to this, countries with a younger population structure will tend to score better in this factor.
Prevalence and impact of chronic pain: Disability Adjusted Life Years (DALYs) resulting from: Back and neck pain; Osteoarthritis; Rheumatoid Arthritis; Migraines and headaches disorders; and Gout. Country-level data, with US cities using state-level data. Source: World Health Organisation — Global Health Observatory, 2016; Institute for Health Metrics and Evaluation (IHME) - Global Burden of Disease Study, 2017.
The combined rates of air, light and noise pollution. The higher the score, the less pollution. Environmental pollution can have significant impacts on the ability to receive adequate sleep duration and quality. The World Health Organisation has warned against the short and long-term effects of excessive environmental background noise (eg. from cars, trains and planes) at night on health and sleep.¹¹
Other research has suggested that light pollution can, among other things, interrupt natural rhythms in the body and delay the release of sleep-inducing hormones¹². Asthma, which can be triggered by exposure to ozone (O3), can seriously impede sleep for sufferers¹³, while other research has suggested a link between exposure to particle pollution in the air (PM2.5/PM10) and sleep apnea (snoring)¹⁴.
Air pollution: Annual median particulate matter (PM2.5/PM10) pollution, and annual median ground-level or tropospheric ozone (O3) pollution at a city level. Sources: AQICN – Air Quality Index historical database, 2019; World Health Organisation – Global Ambient Air Quality Database, 2018.
Light pollution: Average annual night radiance at a metropolitan level, and percent of the urban population exposed to very high artificial night sky light in microcandelas per square metre (>3000 μcd/m2) at a country level. US cities use city-level data. Source: Radiance Light Trends light pollution map, 2016; National Oceanic and Atmospheric Administration - VIIRS Day/Night Band Nighttime Lights, 2018; Falchi, et. al. ‘The new world atlas of artificial night sky brightness’, Science Advances, 2016 (DOI: 10.1126/sciadv.1600377).
Noise pollution: Percent of population exposed to excessive night-time noise (Lnight ≥50dB) and overall background noise population, as well as severity of hearing loss at a city level. Sources: European Environment Agency – noise exposure database, 2019; US Park Service – sound map, 2019; Mimi – World Hearing Index, 2017; Numbeo – pollution index, 2020.
Sleep Duration (minutes)
The average duration of sleep experienced by inhabitants. Sleep researchers advise people to receive a minimum number of hours sleep in order to reduce the likelihood of sleep deprivation. Experts at the US National Sleep Foundation have advised adults receive a minimum of at least seven hours (420 mins) of sleep a night.¹⁵
Mean sleep duration in minutes, per night. Duration estimates modelled using country-level data, with US cities adjusted using county-level data. Sources: OECD Time Use Survey, 2020; “A global quantification of “normal” sleep schedules using smartphone data”, Science Advances journal (DOI: 10.1126/sciadv.1501705), 2016; Economist/Sleep Cycle country sleep study, 2018; Fitbit country sleep duration study, 2019; Jawbone – USA circadian rhythm study, 2014
The percentage of the adult population getting inadequate sleep on any given night.
US cities employ official county-level data on percent of adults receiving under seven hours of sleep. National figures are modeled estimates from various country-level data sources. Sources: Center for Disease Control, 2019; Sleep Cycle, 2020; Financial TImes, 2018; Barmer – Doctor Report, 2018; YouGov.au, 2018; Aviva sleep study, 2016