Sleep Physiology

A while back I promised you that I would give you some good info about sleep, the significance of it, and how to maximize sleep quality any time, any where. As JetBlue crewmembers we already have some good information available to us courtesy of NASA, and more cutting edge info is on the way from our recently created Alertness Management Program, which will focus on issues such as pairing fatigue-analysis, and the possibility of Transcon-turns/longer duty days. Most of us in this line of work have had no choice but to become really good at sleeping any time and any where. Many of us may already consider ourselves sleep experts. I’ll do my best to dig up some new and useful info for you.

We spend on average one third of our lives sleeping, yet why we need sleep and what goes on in our brains while we sleep is still a big mystery to neuroscientists and dream-analysts alike. Scientists do agree that we need sleep for survival but mostly for brain function and restoration, and not so much for our bodies. Muscles and other tissues can repair and grow while physically resting in a wakeful state, however, since most Growth Hormone and other restorative hormones are released during the deepest stage of sleep, a lot growing and repair takes place while sleeping. Every brain requires some amount of sleep and in certain stages, but the requirement varies among individuals and especially among ages. No matter whom you are, or how much caffeine you have just consumed, without sleep eventually your mental performance will continue to degrade to the point of uselessness. After about 18 hours without sleep your reaction time slows from a quarter second to a half second and you may experience bouts of microsleep, zoning out for 2 to 20 seconds. At the 20 hour mark your reaction time slows to the equivalent of a B.A.C. of .08% – DUI criteria in most states. More on the benefits of napping to come, but during periods of sleep deprivation, even a 15 minute nap can restore your alertness and cognizance to functional levels.

Theories of the purpose of sleep currently center on allowing the brain to review and consolidate all the streams of info it gathered while awake, create long term memories and synapse-mapping and strengthening, which make you a better learner with a sharper memory, and, to restore and detoxify the brain. When you sleep you clear your brain of the neurotransmitter known as Adenosine, which causes fatigue. Melatonin, the ‘sleep-inducing’ hormone secreted at night by the pineal gland, is also a powerful antioxidant and immune builder, clearing the brain of free-radicals. All of us dream, (with exception of a select few who have incurred some sort of brain injury), although not everyone remembers their dreams, and certainly not every dream is remembered. Freud may state otherwise but neuroscientists currently believe that the content of our dreams is nothing but gibberish – a random replaying and sorting of the day’s events that is of no significance. So if you thought you had a vision or glimpse into the future, scientists currently disagree with you. The jury is still out on all of this though and besides, your psychiatrist may gain a lot of insight into your psyche from your dream diary.

Sleep occurs in two states: Non-REM sleep, and REM, (Rapid Eye Movement), sleep. There are four stages of Non-REM sleep and one stage of REM sleep. In general Non-REM sleep precedes REM sleep in contains the deepest sleep stages. REM sleep is where most dreaming takes place and as the night progresses this stage gets longer. You dream more in the morning hours and wake more easily. Each 5-stage sleep cycle lasts about 90 minutes and in between these cycles you may sometimes experience restful wakefulness – a normal occurrence and not a sign of insomnia. The first two cycles of sleep have longer and deeper Non-REM stages and shorter REM stages, so you sleep deepest shortly after going to bed and this is when almost all of your Growth Hormone is released. Melatonin levels peak around 2 AM your local time zone and then rapidly taper off as Cortisol hormone levels rise to help you wake up. It is important that your brain experience some amount of sleep in all 5 stages for proper restoration and function, but if you wake up extra early one morning and miss most of your REM sleep, you’ll be fine.

Our sleep cycle is set based on a 24 hour, day/night cycle also known as circadian rhythm. Two hormones, Melatonin and Cortisol, directly affected by sunlight, guide your circadian rhythm. We are meant to sleep at night, and this isn’t just because in ancient times there was no light to see by at night. Sunlight naturally blocks the production of melatonin via your optic nerve and a bundle of nerves, (The SCN bundle), within the hypothalamus. Melatonin makes us feel ‘sleepy’ and helps us drift off to sleep. Cortisol is secreted by the Adrenal glands, peaks in the morning and helps wake us up. This is a natural and healthy cycle of Cortisol production at normal levels. (Cortisol is being labeled lately as the “stress hormone”, and is secreted in much larger, unhealthy levels during times of stress.) Body temperature, blood pressure and metabolism are also affected by circadian rhythm, lowering at night. Your pineal gland does not care how many naps you may have had earlier that day or if you’re currently in the middle of a redeye, it will secrete melatonin regardless according to schedule, about one hour before your normal bed time, with levels peaking around 2 AM your home-time zone. A caffeinated beverage at this point can help you stay alert by blocking Adenosine receptors in the brain, but will not block melatonin. Turning on the cockpit dome lights to bright can help suppress melatonin levels, staving off sleepiness. Paradoxically, if you spent the morning before your redeye playing outdoors, you could experience higher levels of melatonin than you normally would, because sunlight promotes serotonin production – the ‘happy and pleasant’ neurotransmitter - and at night, your brain converts serotonin into melatonin. Melatonin is only produced at night around your normal bed time…set by your circadian rhythm. On a redeye of course you want to suppress melatonin, but be sure to get some rest when you land. Melatonin clears the brain of Adenosine, detoxifies it, and boosts your immune system, so you don’t want to continually suppress it or repeatedly sleep counter to your circadian rhythm. This could make you prone to Chronic Fatigues Syndrome.

Our body clocks and circadian rhythm can be adjusted about only two hours each day, which means if you’re west-coast based and flew to Orlando for your PC it will take at least two days to adjust to east coast time…right about when it is time to fly home again! This time zone/circadian rhythm adjustment period is called ‘Internal Desynchronization’, more commonly known as jet lag, and melatonin secretion is disrupted in its timing and duration, leading to sleep disturbances and insomnia. This is something we as pilots are expected to accept as part of the job although it can adversely affect our performance. Perhaps the new Alertness Management Program will address ‘Internal Desynchronization’ as an occupational hazard, and provide insight and coping measures, along with pairings designed to minimize fatigue while maximizing productivity. Of course this is a lot to ask for!

When we feel sleepy during the day this could be caused by a number of things, but usually not melatonin, since it is only released at night and is blocked by sunlight, (unless of course you took too much supplemental melatonin which can remain in your system and cause you feel groggy after waking). Mental fatigue during the day is most likely caused by a build of the neurotransmitter Adenosine. A short nap or a caffeinated beverage can fix this by clearing adenosine or blocking its brain receptors, respectively. Also, our brains are huge power plants, consuming up to 20% of our energy production. This energy comes from glucose, (blood-sugar), and when levels run low, we experience mental fatigue among other possible hypoglycemic symptoms. Eating complex carbs for sustained energy or having a quick carb snack should fix this. When we eat a high-carb/high-calorie meal however, sometimes this can cause higher levels of the amino acid Tryptophan to cross the blood-brain barrier. Here comes the sunlight paradox again: Although sunlight blocks melatonin production it also uses tryptophan to produce the serotonin mentioned earlier, making us feel pleasant, and so peaceful that we can easily become sleepy. Large meals also require a lot of energy for digestion and cause fluctuations in blood-sugar levels, directly affecting our alertness and mood. Eating balanced, smaller and more frequent meals should keep you off the blood-sugar roller coaster.

Current sleep lab studies recommend about 8 hours of sleep a night, but this amount varies greatly among individuals. The older we get we don’t sleep as deep, as long, and wake more throughout the night. This is mostly due to a gradual decrease in all hormone levels as we age. Physical activity, especially outdoors, can up your requirement and help you sleep deeper. More discussion to come on sleep-hygiene and on how to get a ‘good night’s sleep’. If you run up a sleep deficit you can usually catch up with one full night of sleep on your normal schedule. However, your circadian rhythm always determines when you will have the best quality sleep, so trying to make up for sleep deficit on your days off by sleeping much later than normal may only confuse your circadian rhythm further. Interesting enough to note is how our bodies/brains will eventually adjust and compensate to a certain degree to continued reduced sleep times. Eventually it seems we compensate automatically to reduced rest times, by sleeping deeper, in effect giving us adequate rest for a less amount of time in bed.

More to come on nap-taking strategies, sleep-hygiene, and natural and herbal sleep-aid supplements. Stay posted.

Sources: Sleep Disorders by H. Ross, K. Brenner, and B. Goldberg; Time magazine, “The New Science of Sleep”, Dec. 20^th 2004 issue, pages 46-56; From Fatigued to Fantastic!, by J. Teitelbaum, M.D.

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