With another long singlehanded passage about to start, I've been reading about the science of sleep, to see how I might be able to improve my methods of watchkeeping. I've learned a couple of interesting new things:
- Some birds and aquatic mammals can sleep with one side of their brain, whilst the other side is awake and functioning (and then they swap sides, obviously). This is what makes long distance migration possible, and also has a role in protection against predators - an eye on one side can be open and functioning whilst the other is closed and asleep. Now wouldn't that be a fine thing, to have a port and starboard watchkeeper combined in one person? But unfortunately, our physiology doesn't permit it.
- The larger ruminants - horses, cattle, elephants, giraffes - only need 3 to 4 hours of true sleep. This may be because they spend a large part of the day in a state of drowsiness. It agrees with advice that I remember receiving a long time ago, regarding sailing singlehanded: "whenever there is nothing else you need to be doing (keeping a look out, sailhandling, navigating, eating, maintenance), lie down and close your eyes". You can get an appreciable amount of stage one NREM sleep that way.
- REM sleep has an important role in processing and consolidating memories. I have found that after a coastal passage that required alertness and catnapping all the way, I have come into anchorage, anchored, written up the log, eaten a meal, and then turned in for a "catch-up" sleep; then several hours later, I have woken up without the slightest idea where I was! It would take a careful study of the log to work it out. I put this down to having missed my REM sleep.
The sleep cycle goes like this:
Non-REM sleep
Stage N1 (Transition to sleep) – This stage lasts about five minutes.
Eyes move slowly under the eyelids, muscle activity slows down,
and you are easily awakened.
Stage N2 (Light sleep) – This is the first stage of true sleep,
lasting from 10 to 25 minutes. Eye movement stops, heart rate slows,
and body temperature decreases.
Stage N3 (Deep sleep) – You’re difficult to awaken, and if you are awakened, you do not adjust immediately and often feel groggy and disoriented for several minutes. In this deepest stage of sleep, brain waves are extremely slow. Blood flow is directed away from the brain and towards the muscles, restoring physical energy.
REM sleep
REM sleep (Dream sleep) – About 70 to 90 minutes after falling asleep, you enter REM sleep, where dreaming occurs. Eyes move rapidly. Breathing is shallow. Heart rate and blood pressure increase. Arm and leg muscles are paralyzed. During REM sleep, you are close to being awake, and would respond to an alarm.
The sleep cycle of 90 to 120 minutes repeats four to six times over the course of a night. The first two cycles are much the most valuable, containing the most Stage N3 sleep. This explains to me why the four hour watch below has proved to be the most effective, fitting the two most valuable sleep cycles into 3 1/2 hours, with time to go to sleep and wake to a state of alertness before your watch on deck begins.
Thinking about the singlehanded watchkeeper, it seems that the maximum time that you can catnap, and expect to waken reliably to an alarm, is 30 minutes (stages N1 and N2 sleep). I find that I can respond to an alarm set to one hour during the first part of the night, but then will tend to sleep though it, and wake up refreshed after three hours, but feeling guilty for not having kept a lookout. It would be much better to set an alarm for 90 minutes.
There's no getting away from the fact that, in the long term, you need two periods of 90 uninterrupted minutes of sleep in each 24 hours, to avoid sleep deprivation. You simply have to have that Stage N3 deep sleep. Trying to stay awake, taking caffeine or pills, or even continuous catnapping, doesn't work in the long run - the body has to give in to deep sleep eventually, and will sleep longer and deeper as a result.