This release provides raw ambient light data from the physical activity monitor (PAM) component for the combined samples from NHANES 2012 and the NHANES National Youth Fitness Survey (NNYFS). For this component, participants were asked to wear the PAM for seven consecutive days to collect objective information on 24-hour movement when awake and asleep.
Measurement of physical activity with accelerometers had been implemented in NHANES 2003-2006. It was reintroduced to NHANES in 2011-2014 and was included in the 2012 NNYFS as well. The PAM used in 2012 NNYFS and NHANES 2011-2014 measured acceleration 80 times a second (80 Hz) and ambient light (lux) once a second (1 Hz). The collected measurements have been summarized over each minute, hour, and day for each participant and made available in separate files for the NHANES 2012 and NNYFS combined sample (i.e., C_PAXHD, C_PAXMIN, C_PAXHR, C_PAXDAY). This release provides the raw lux data collected through the PAM component.
Lux is the International System of Units (SI) derived unit of illuminance. One lux is equal to one lumen per square meter. Ambient light, measured here in lux, may affect subject sleeping habits and could be a useful tool in analyzing circadian rhythms and sleeping patterns. Some estimates of lux values are shown in the following table (ActiGraph, 2013). NOTE: These levels are only estimates and are not meant for exact interpretation of the light detected by the device. Lux values reported by the PAM devices are only intended as a general guideline for average light intensities.
| Lux Level | Interpretation Comparison |
|---|---|
| 1 | Twilight |
| 50 | Family living room |
| 100 | Very dark overcast day |
| 320-500 | Office lighting |
| 400 | Sunrise/sunset |
| 1,000 | Overcast day |
| 10,000-25,000 | Full daylight |
Datasets included in this release were comma-separated-value (.csv) text files compressed using the bzip2 format to reduce file sizes. There is one zipped archive file (tar.bz2 format) for each participant. As bzip2 is a freely available open-source file compression program, these zipped archive files can be uncompressed using standard compression software (e.g., Winzip) or with common compression utilities available in R and Python.
In 2012, all participants aged 3 years and older were asked to wear a PAM for 7 consecutive days.
Participants were asked to wear the PAM starting on the day of their exam in the NHANES Mobile Examination Center (MEC), to keep wearing the PAM 24 hours a day for the next 7 days, and to remove it on the morning of the 9th day. The device used in NHANES was the ActiGraph model GT3X+, manufactured by ActiGraph of Pensacola, FL (ActiGraph, 2013). The devices were programmed to begin detecting and recording acceleration and ambient light at the end of the participant’s MEC exam session and to stop recording data eight days later (i.e., on the 9th calendar day). The device was placed on a mesh wristband (like a watch), custom fitted for each participant’s wrist and worn on the non-dominant hand, if possible. The participant was told not to do anything with the device except to wear it. If the participant needed to remove it, for any reason, he/she was instructed to put it back on the same wrist in the same orientation, as soon as possible.
A toll-free number and information materials describing the PAM were given to the participant. No activity logs, diaries, or records were kept by the participant. The PAMs were returned by mail in postage-paid padded envelopes provided to each participant.
Detailed descriptions of the protocol are provided in the PAM Procedure Manual available on the NHANES and NNYFS websites.
Survey staff completed an examiner training program that covered the basic operating features of the PAM, equipment operation, subject recruitment strategies, PAM device initialization, and troubleshooting. Field performance and response rates were monitored by NCHS and contractor staff.
Not every eligible participant has a data file. For example, if a participant refused to wear the PAM, did not return it, or the data could not be retrieved from a damaged PAM, then the participant does not have a data file. Per the protocol, the first day and last day of data collection for each participant are partial days.
The lux files (one per participant) use the following naming convention:
[SEQN]_Lux.csv
where SEQN is the respondent sequence number of the participant.
The lux file for each participant contains the following two fields:
HEADER_TIMESTAMP (Date and time stamp)
A datetime in the following format: YYYY-MM-DD HH:MM:SS, where YYYY-MM-DD represents a date (e.g., “2000-01-03”) and HH:MM:SS represents a time.
LUX (Lux value)
Measure of ambient light in lux (i.e., lm/m2; lumen per square meter). Values at or above 2,500 lux were coded as “2,500 lux”.
The analysis of the combined NHANES and NNYFS PAM data should be conducted using the appropriate survey design variables, sample weights, and the basic demographic variables. Please refer to the NNYFS Analytic Guidelines for further details on the use of sample weights and other analytic issues.
The PAM component was reintroduced to NHANES in 2011-2014 and was included in the 2012 NHANES National Youth Fitness Survey (NNYFS) as well. A total of 16,417 persons ages 3 years and older wore the PAM during these surveys and had data available for analysis: 2011-2012 NHANES participants 6 years and older (n=6,917); 2013-14 NHANES participants 3 years and older (n=7,776); and 2012 NNYFS participants 3-15 years (1,477). In addition, PAM data were collected for 3-5 years old participants in the 2012 NHANES (n=247) as well. However, due to disclosure concerns, these data are only available in the NCHS Research Data Center (RDC). Overall, 96% of participants with data wore the PAM until the 9th day. About 2% of the participants with PAM data wore it for less than 7 days. On the non-partial days (that is, not the first or last days of wear) the mean number of valid minutes per day was ~1,437 minutes out of a possible 1,440 valid minute.
In the NHANES/NNYFS Combined Sample, PAM data is available for 2,762 participants 3-15 years old. The current release (C_PAXLUX) consists of 2,762 zipped archive files – one for each participant. As previously noted, the data is available through the NCHS RDC.