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Circadian Rhythms, Sleep, and Cognitive Skills: Evidence From an Unsleeping Giant

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Demography

Abstract

This study analyzes the effects of sleep duration on cognitive skills and depression symptoms of older workers in urban China. Cognitive skills and mental health have been associated with sleep duration and are known to be strongly related to economic behavior and performance. However, causal evidence is lacking, and little is known about sleep deprivation in developing countries. We exploit the relationship between circadian rhythms and bedtime to identify the effects of sleep using sunset time as an instrument. Using the Chinese Health and Retirement Longitudinal Study, we show that a later sunset time significantly reduces sleep duration and that sleep duration increases cognitive skills and eases depression symptoms of workers aged 45 years and older. The results are driven by employed individuals living in urban areas, who are more likely to be constrained by rigid work schedules. We find no evidence of significant effects on the self-employed, non-employed, or farmers.

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Notes

  1. The data are available online (http://www.csrs.bj.cn/society.aspx).

  2. The introduction of portable devices, such as smartphones and tablets, is associated with shorter sleep duration and insomnia problems (Fossum et al. 2014).

  3. Author’s own calculation based on National Bureau of Statistics of China (2014). For the provinces having at least one-half of the areas located within UTC+8 longitudes, we count 50 % of their population and GDP; for Hunan, we count 30 % of its population and GDP.

  4. It is worth reminding that sunset differences are not only determined by the longitude of a location but also by the latitude, with northern cities having longer days in the summer and shorter days in the winter with respect to cities in the south of the country.

  5. Details of the plan are available online (http://www.gov.cn/2011lh/content_1825838.htm).

  6. For a list of provinces by region, please see National Bureau of Statistics of China (2012).

  7. More light in the evening has been also associated with lower levels of crime and car accidents (Doleac and Sanders 2015; Smith 2016).

  8. CHARLS adopts multistage stratified PPS sampling.

  9. Huang et al. (2013) provided an extensive description of the data.

  10. Information on sleep duration is missing in 10 % of the sample. To avoid double selection due to the missing responses in both the sleeping and cognitive tests variables, we impute the missing information on sleeping behavior using the standard Stata routine to impute missing variables values and individual information on gender, age, education, employment status, province of residence, rural status of residence, and interview wave. We use Stata command mi impute to impute the missing sleeping hours. The imputed sleeping hours used in the regressions is the mean of 100 rounds of nonnegative imputations. Although point estimates are substantially identical, the imputation procedure has a cost in terms of precision of the first stage (standard errors are larger). However, reduced-form estimates are less noisy (and less selected, by definition), so our 2SLS estimates are more precisely estimated (see Table S3 in Online Resource 1 for a comparison).

  11. The calculator is available online (http://www.esrl.noaa.gov/gmd/grad/solcalc/).

  12. By restricting our sample to areas located in UTC+7, UTC+8, and UTC+9, we dropped 106 and 102 individuals in 2011 and 2013, respectively.

  13. Data are available online (http://www.who.int/countries/chn/en/).

  14. However, these restrictions do not significantly affect our results. Restricting the sample to individuals younger than 60 yields very similar results (available upon request).

  15. This sample consists largely of individuals who retired from the labor force. As a placebo test, we consider the self-employed and farmers separately in our robustness checks.

  16. Using an overall measure of respondents’ cognitive function (0–21)—which sums the scores obtained in the TICS test, the Memorial (word recall) test, and the Draw test—we find that a 15-minute increase in sleep duration would increase the overall cognitive score by 7.5 % with respect to the mean of the dependent variable (results are available upon request).

  17. We also find no evidence of significant effect on drawing skills. The Draw test examines the ability to redraw a picture of two overlapping pentagons. Respondents score 1 if the task is successfully performed, and 0 otherwise. Results are available upon request.

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Acknowledgments

We are thankful to James Fenske, Daniele Paserman, and Guglielmo Weber for their comments and suggestions. We also thank seminar attendees at the University of Oxford and at the Conference on Health, Demography and Ageing in China, Stanford Center for International Development, October 2015.

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Correspondence to Osea Giuntella.

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Giuntella, O., Han, W. & Mazzonna, F. Circadian Rhythms, Sleep, and Cognitive Skills: Evidence From an Unsleeping Giant. Demography 54, 1715–1742 (2017). https://doi.org/10.1007/s13524-017-0609-8

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