Rules for Extracting Sleep Quotas
Here are the rules we followed for extracting sleep quotas when they were not explicitly given by the authors/investigators:
- Generally, we searched the papers for a table that had values for TS (total sleep), PS (paradoxical sleep or REM), and QS (quiet sleep or NREM sleep). While some papers had tables, we had to make sure they did not mix adult and juvenile results. We always sought to identify the age at which sleep recordings were performed.
- When sleep quotas were not explicitly given in the paper under consideration, we followed the following procedure: We first calculated TS. There were a number of different ways
that we could obtain the total sleep value. If TS was given, we only had to make sure that it was given in hours and the recordings were for a 24-hour period. If only the amount of wakefulness was given, we converted this value to TS by subtracting this number from 24 hours.
- Once we had calculated TS, the next value that we sought to calculate was PS. This data was either given in minutes, hours, % of 24 hours, or % of TS. If no PS values were given, we proceeded to calculate NREM values first, though this was very rare.
- After we had calculated TS and PS numbers, we found QS by subtracting PS from TS. PS plus QS should add up to TS. If we had just calculated QS, there was a chance that QS and PS would not add up to 24 hours due to inaccuracies in the data.
- When sleep recordings were less than 24 hours but at least 12 hours, we found that the obtained values (given by the investigators) were reliable estimates of daily sleep quotas.
- When sleep recordings were less than 12 hours, we flagged the obtained values as potentially unreliable estimates. There were 22 such data points. Click here to view those papers with recordings times less than 12 hours.
Certain animals (mostly ungulates) exhibit an intermediate form of sleep called drowsiness. We followed Zepelin’s rules for calculating drowsiness when they were not explicitly given by the authors. Since so few animals in our initial database exhibited drowsiness, it could not be accurately calculated in the remaining animals so we set this aside in the initial database; eventually, we will need to calculate it.
Special rules for sea mammals who display unihemispheric sleep patterns
Terrestrial, placental mammals may be the only animals with bi-hemispheric sleep (i.e., the same type of sleep occurring simultaneously in both hemispheres). Marine mammals, birds, and some reptiles all display some form of unihemispheric sleep (i.e., one form of sleep occurring in a single hemisphere at a time). We therefore have to devise special scoring systems for unihemispheric forms of sleep.
Members of three different orders that contain aquatic mammals - cetaceans (dolphins, porpoises, and whales), carnivores (seals, sea loins, and otters), and sirenians (manatees) typically engage in unihemispheric sleep. Cetaceans exhibit a clear form of unihemispheric SWS. EEG signs of REM are absent, but cetaceans show other behavioral signs of REM including rapid eye movements, penile erections, and muscle twitching. The two main families of Pinnipeds, Otariidae (sea lions and fur seals) and Phocidae (true seals), show both unihemispheric and bihemispheric forms of sleep. Phocids sleep under water (obviously holding their breath) while both hemispheres exhibit either REM or SWS. Amazonian Manatees (Trichechus inunguis) also sleep while underwater exhibiting three sleep states: bihemispheric REM, bihemispheric SWS, and unihemispheric SWS.
Both hemispheres of the brain awaken to surface and to breathe. Sleep deprivation in an animal exhibiting unihemispheric sleep may evidence unihemispheric sleep rebound, prompting some authorities to claim that sleep can serve a primary function for one part of the brain rather than another, regardless of the state of body. These data underline the importance of looking for potential relationships between duration of sleep states and particular brain regions. The data on unihemispheric sleep in marine mammals also suggests that REM and NREM serve distinct functions as animals without full polygraphic REM can survive. In addition, when REM occurs in marine mammals, it, apparently, is always bihemispheric. The bilateral nature of REM may be considered one of its costs, and the brain structure of certain marine mammals, apparently, cannot bear these costs.
To score sleep quotas in marine mammals, we need a special system. We need percent of total sleep time for each of the following: unihemispheric NREM (or USWS); bihemispheric NREM; unihemispheric REM (if it ever occurs at all) and bihemispheric REM. To get a total for unihemispheric NREM, we had to sum across right and left hemispheres. In addition, all of these percents had to be converted to hours so that comparisons could be made with sleep quotas of terrestrial mammals.
In addition, some marine mammals will sometimes have two values for sleep: on land and in water. These numbers had to be averaged together.
We also want to score another peculiarity of sleep in marine mammals: unilateral eye closure (UEC). Many of these animals sleep with one eye open. For UEC, we simply scored whether UEC was present in the species when they sleep and whether the open eye was contralateral to sleeping hemisphere greater than 50% of the sleep time.