How the Brain Sleeps—and Why it Matters Offshore (2 of 3 on Sleep)

Learning Objectives:

1)    Identify stages of sleep

2)    Learn when to safely wake from sleep

3)    Understand that sleep is controlled by two mechanisms: the sleep homeostat and the circadian oscillator.

4)    Learn how to control the pressure to go to sleep.

The first time I woke abruptly offshore and could not immediately understand what I was seeing, where I was or what I was doing. It was deeply unsettling. I was awake, upright, and scanning the horizon, yet my brain lagged behind reality. 

Sleep is not an on/off switch. It is a structured biological process composed of multiple stages. Two sailors may both be asleep, but one can wake instantly while the other may be neurologically unreachable for minutes. What is the difference and how can we learn how to use these differences to our benefit? 

A key transition during sleep occurs at the interface between the thalamus and cortex. During wakefulness, sensory information flows freely through the Thalamus to the Cortex (the area of the brain where we perceive, think, evaluate and learn). As sleep deepens, this thalamic gate closes and becomes harder to open. Signals still arrive in the Thalamus but are not transferred to the cortex.

Stages and depth of Sleep and why this knowledge is important 

Sleep occurs in cycles consisting of non‑rapid eye movement (NREM) sleep followed by rapid eye movement (REM) sleep. Each stage of NREM sleep lasts roughly 20 minutes and becomes progressively deeper. A full sleep cycle consists of about 80 minutes of NREM sleep followed by 10 minutes of REM sleep, in aggregate taking 90 minutes. Cycles of sleep are repeated during the normal nighttime to result in a full night of 7-8 hours of refreshing sleep. In a sleep cycle one enters into a shallow NREM period (~20 minutes duration) in which one is easily awoken AND importantly, from which one can immediately “think” and “perform”. With further time in NREM the sleep deepens and it becomes harder to be awoken. If you are awoken, you now have what we call sleep inertia. One is confused, and its difficult to think and perform for a while. Following NREM sleep we enter into REM sleep. After REM sleep is complete, we have gone through one sleep cycle and commence the second sleep cycle by entering into light NREM sleep and so forth and so on.  

Waking from deep NREM sleep produces sleep inertia—a period of impaired cognition, slow reaction times, and poor judgment. Offshore, this can be more dangerous than moderate sleep deprivation. But waking from shallow NREM sleep allows one to instantly execute cognitive functions. This is counterintuitive. We function better after a shorter nap. 

REM sleep stabilizes perception. Without it, dreamlike experiences intrude into wakefulness, producing hallucinations. Some sailors tolerate REM deprivation better than others. Knowing your own vulnerability matters.  Dream‑like experiences intrude into wakefulness, producing hallucinations. Some sailors tolerate REM deprivation better than others. Knowing your own vulnerability matters. 

Understanding sleep architecture allows us to design offshore sleep strategies that preserve vigilance while minimizing risk. Since we can’t sleep for 7-8 hours and keep an effective lookout nor navigate and optimize sailing performance, can we divide a night of sleep into smaller naps, and how do we do this so as to wake with the minimum sleep inertia? This is the key question to be answered to allow an effective nap schedule for shorthanded offshore sailors.  

The Sleep Homeostat and the Circadian Oscillator 

The 24 hour day of cycles of wakefulness and sleep are regulated by two opposing brain control systems. We all recognize them, even if we don’t understand them. But by understanding these control systems together with our knowledge of sleep stages we will be able to effectively design nap strategies for shorthanded offshore sailing.  

The first system is the circadian oscillator which is present in every cell in the body. It oscillates every day and leads to daily oscillations in the genes that cells express. The oscillation is not exactly 24 hours. If left in total darkness our oscillator will cycle once every 24.5 hours. But, the clock is entrained by our normally daily light and activity cycle to our 24 hour day. We all experience the circadian oscillator when we travel across time zones and experience jet lag. We might wake up at 3am in the new time zone because our circadian clock has not changed to the new time zone and is telling us to wake up when we want to sleep. The clock can be shifted by about an hour per day in the new time zone as it is entrained by signals such as the timing of daylight.  If you fly east over six time zones it will take several days for your circadian oscillator to be entrained to the new time (just in time to fly home).

The second system is the sleep homeostat which measures, or integrates, the amount of time that we have been awake. The longer we are awake, the more sleep pressure or sleep drive that the homeostat provides. By the end of a normal day the homeostat and the circadian oscillator are in phase: the homeostat is providing sleep pressure (go to sleep) and the circadian oscillator is no longer trying to keep us awake. The result, we can enter easily into a night of sleep.  

For the offshore sailor understanding the sleep homeostat is what is most important since most of today’s fastest race boats don’t cross enough time zones per day for jet-lag to be a problem.

However, it should be noted that really fast race boats could encounter sail-lag, as I will call it. In the Southern ocean at 45degrees south the surface of the earth is about 15,295nm. Each 1 hour time zone is a distance of 637nm. To cross a time zone in 24 hours and begin to experience sail-lag one would have to sail at an average speed of 26.5 knots. In the 2025/26 Jules Verne Trophy by Sodebo the sailed at an average speed of 27.17 kts. In the southern ocean where there was mostly reaching and downwind sailing their average speeds would have been higher. It will be interested to see if sail-lag becomes a consideration in future races with even faster technology. 

Consider the sleep homeostat to be a balloon. In the morning after a full night of sleep the ballon is empty of air and there is no pressure (sleep pressure). As we wake and go about our day, the balloon continuously fills with air and by the end of the evening has significant pressure to help drive us to sleep. Every minute that we are asleep the pressure in the balloon is declining. By the next morning the pressure has returned to zero, assuming that we got a full night of sleep.

It may be clear from this discussion that the one thing we can purposefully control is sleep pressure. We can consciously control how much “air” is in the balloon by sleeping. Remember, ever minute asleep reduces sleep pressure and every minute awake increase this pressure. It should be clear then, that naps with brief awakenings are the key to safe shorthanded sailing to minimize sleep pressure and to manage the untoward consequences of sleep deprivation.  

In the next blog of the sleep series I discuss Polyphasic Sleep and How to Nap Offshore 

Sailor Takeaway 

·                Sleep has stages; waking from deep sleep is dangerous. 

·                Light sleep allows rapid arousal. 

·                Sleep inertia can impair judgment offshore. 

·                REM sleep stabilizes perception. 

·                Sleep homeostasis and circadian oscillator regulate sleep and wake 

·                The sleep homeostat integrates time awake and provides the pressure to sleep 

·                We can regulate sleep pressure by carefully controlling nap schedules and durations