I am working on the MESA actigraphy data. And I found that the average sleep onset latency in the summarised dataset is around 5 min, which is much shorter than the average SOL reported for this age range (~15 min). I read the MESA actigraphy scoring manual, and I think the short SOL may be related to how scorers defined bedtime when cleaning actigraphy data.
Do you guys know who on the MESA team I should contact and ask about their bedtime definition?
Good question - sleep onset latency is difficult to discern from actigraphy alone. The reason it is ~5 minutes in MESA actigraphy data is because the Actiware scoring software used a sleep onset detection method of "5 minutes of immobile time". Scorers generally set bedtime (i.e., the start of an Actiware REST interval) around the time the activity signal diminished, which typically corresponded to what ActiWare considers "immobile time" (activity counts < 2 at 30-second epochs).
We don't have any additional information to provide beyond what is available on NSRR.
Another approach to consider is to recompute onset latency from the epoch-by-epoch data using different criteria.
Thank you very your quick response! I am a little confused by the actiware onset detection of "5 minutes of immobile time". For example, if there is 5 minutes of immobile time from the start of REST interval, then the SOL is 0 min? I am confused because the mean is actually less than 5 min (4.8 min), and there are subjects with 0 min SOL.
Thanks, you're right, I should have clarified further. The "immobile time" counting is done on a continuous basis by the Actiware software, so if there were 5 minutes of immobile time before the beginning of a REST interval this can result in a value of 0 minutes of sleep latency.
I think the point I was trying to get at is that sleep latency in actigraphy was most commonly in the 0-10 minute range because the scoring rules called for REST intervals to be set approximately around the time activity counts started to decrease (in other words, around the time the subject started becoming more immobile than mobile). People are typically lying still when they start to try and fall asleep, yet (of course) the actigraphy device can't tell when the person truly transitions from wake to sleep.
The scorers did reference self-reported sleep diaries in the setting of REST intervals, however this was just one input used (others being: activity count levels; light levels; event markers). Perhaps if the sleep diary alone was used in the setting of REST intervals we might get "truer" estimates of sleep onset latency. For instance, a subject reports they got in bed and tried to sleep at 10 p.m., yet the actigraphy device still shows sporadic movement until 10:30 p.m., when the subject triggered the "5 minutes of immobile time" for sleep onset, thus giving ~30 minutes of sleep onset latency for that night.
Bottom line: sleep latency and actigraphy is very tricky!
Any chance that we can get sleep diary data from MESA? or should I contact the MESA center? We are looking at different "bedtime" definitions, and would like to use MESA data as an example.
Unfortunately, the MESA (Exam 5) sleep journals were never entered electronically. ☹️
They were filled out on paper and scanned to PDF at the field sites. The actigraphy scorer kept the PDF open for review while scoring.