Association Between Sleep Duration and Cognitive Decline

This pooled cohort study investigates the association between sleep duration and cognitive decline in a combined analysis of 2 nationally representative aging cohorts.


Sleep duration
In the ELSA, individuals were asked to report "How many hours of sleep do you have on an average week night? (You can tell me to the nearest half hour)".
In the CHARLS, individuals were asked to report "During the past month, how many hours of actual sleep did you get at night (average hours for one night)? (This may be shorter than the number of hours you spend in bed)".

Cognitive assessment
Cognitive assessment was conducted in face-to-face interviews and covered three domains, memory, executive function and orientation. The ELSA and the CHARLS used the same methods to assess memory and orientation.

Memory
The interviewers read out a set of 10 words "at a slow steady rate approximately one word every 2 seconds" and then ask the individual to "recall aloud as many of the words as you can, in any order". Enough time was allowed to recall, approximately up to 2 minutes. After questions and tests on other aspects, the individuals were again asked to recall the words as many as they could.

Executive function
In the ELSA, individuals were asked to "name as many different animals as you can think of" in one minute.
In the CHARLS, individuals were asked to answer "What does 100 minus 7 equal? And 7 from that? And 7 from that? And 7 from that? And 7 from that?" In addition, individuals were requested to see and draw the following picture.

Orientation
The individuals were asked to report the "today's date (the day of month, month, year)" and "what day of the week it is today".

Covariates
All the covariates were collected in face-to-face interviews unless otherwise stated.

Age
Individuals were asked to report "date of birth".

Sex
Sex were coded with or without asking.

Education
In the ELSA, individuals reported their "highest educational qualification" in a self-completion questionnaire, where education level was classified as no qualification, level 1 national vocational qualification (NVQ) or certificate of secondary education, NVQ2 or general certificate of education (GCE) O-level, NVQ3 or GCE A-level, higher qualification but below degree, and degree level or higher or NVQ4/5. High level of education was defined as ≥ NVQ3 or GCE A-level.
In the CHARLS, individuals reported their "the highest level of education completed" in interview. Education level was classified as no formal education (illiterate), did not finish primary school but capable of reading and/or writing, sishu/home school, elementary school, middle school, high school, vocational school, two-/three-year college/associate degree, four-year college/Bachelor's degree, Master's degree, Doctoral degree/Ph.D. High level of education was defined as ≥ high school.

Cohabitation status
Cohabitation status indicated currently living alone or not. Individuals in both cohorts were asked to report their "current legal marital status". Common-law marriage was considered as married.
In the ELSA, individuals chose from "single (that is never married), married (first and only marriage), a civil partner in a legally-recognized civil partnership, remarried (second or later marriage), legally separated, divorced, and widowed". Living along was defined as single, legally separated, divorced, or widowed.
In the CHARLS, individuals chose from "married with spouse present, married but not living with spouse temporarily for reasons such as work, separated, divorced, widowed, and never married". Living along was defined as separated, divorced, widowed, or never married.

The Center for Epidemiologic Studies Depression Scale (CES-D)
The ELSA used an eight-item version of the CES-D. The individuals answered the following eight questions with Yes or No for much of the time during the past week.
(2) You felt that everything you did was an effort?
(4) You were happy? The CHARLS instead used a ten-item version of the CES-D as below.
(1) I was bothered by things that don't usually bother me.
(2) I had trouble keeping my mind on what I was doing.
(4) I felt everything I did was an effort.
(5) I felt hopeful about the future.
(10) I could not get "going." The individuals were asked to choose one of the following responses for each item above that happened during the last week.
(1) Rarely or none of the time (< 1 day) (2) Some or a little of the time (1-2 days) (3) Occasionally or a moderate amount of the time (3-4 days) (4) Most or all of the time (5-7 days)

Body mass index
Body mass index was calculated with interviewer-performed physical examination and the following formula: weight (kg) / height 2 (m 2 ).

Systolic blood pressure
Systolic blood pressure was the mean of three measures of systolic blood pressure.

Current smoking
Individuals were asked to answer "Do you smoke cigarettes at all nowadays" in the ELSA or "Do you still chew tobacco, smoke a pipe, smoke self-rolled cigarettes, or smoke cigarettes/cigars?" in the CHARLS with Yes or No.

Alcohol consumption
Individuals in the ELSA chose the frequency of alcoholic drinking in the past 12 months in a self-completion questionnaire. Twice a day or more, daily or almost daily, once or twice a week, once or twice a month, special occasions only, or, not at all?
In the CHARLS, alcohol consuming involved the following three questions: (1) How often did you drink liquor, including white liquor, whisky, and others per month in the last year?
(2) How many times per month did you drink beer in the last year?
(3) How often did you drink wine or rice wine per month in the last year?
The individuals were asked to choose one of the following responses for each item above.
(1) Once a month (2) 2-3 times a month (3) Once a week (4) 2-3 times a week (5) 4-6 times a week (6) Once a day (7) Twice a day (8) More than twice a day Alcohol consuming in the CHARLS was defined as at least once a week for whichever of the above three questions.  3.10 Diabetes, coronary heart disease, stroke, cancer, asthma, and chronic lung disease Individuals were requested to confirm whether they had been told by a doctor that they had the following conditions: (1) diabetes (diabetes or high blood sugar) (2) coronary heart disease (angina or heart attack in the ELSA; heart attack, coronary heart disease, angina, congestive heart failure, or other heart problems in the CHARLS) (3) stroke (4) cancer (cancer or malignant tumour) (5) asthma (6) chronic lung disease (such as chronic bronchitis or emphysema) In addition, confirmation of current anti-diabetic therapy was included in the definition of diabetes. a Data are presented as LSM and 95% confidence interval after adjusting for age, education, body mass index, the Center for Epidemiologic Studies Depression Scale score, systolic blood pressure, cohabitation status, current smoking, alcohol consumption, diabetes, coronary heart disease, stroke, cancer, chronic lung disease, and asthma.  a Data are presented as LSM and 95% confidence interval after adjusting for age, education, body mass index, the Center for Epidemiologic Studies Depression Scale score, systolic blood pressure, cohabitation status, current smoking, alcohol consumption, diabetes, coronary heart disease, stroke, cancer, chronic lung disease, and asthma. a Data are presented as LSM and 95% confidence interval after adjusting for age, education, body mass index, the Center for Epidemiologic Studies Depression Scale score, systolic blood pressure, cohabitation status, current smoking, alcohol consumption, diabetes, coronary heart disease, stroke, cancer, chronic lung disease, and asthma.