Now let's move into the credit/debit system for understanding the sleep/wake… — Page 210, Lesson 336

I want to walk you through the relationship between time of day and accidents, and then into the sleep/wake credit and debit system, followed by how we measure sleep and its stages. First, researchers have looked at whether there's a link between the time of day and when accidents happen. They found that driving accidents peak at certain times — for example, 1500 hours, which is 3 PM. But they also note that other factors like traffic density and road conditions affect those results, so it's not purely about the clock. In aviation specifically, the time of day has been identified as a causal factor in a number of incidents. Now let's move into the credit/debit system for understanding the sleep/wake cycle. Think of it like a bank account. For every hour you spend asleep, you earn two points — that's credit. For every hour you spend awake, you lose one point — that's debit. This is only a rough measure, because individuals vary considerably in how much sleep they actually need. There's a limit to how much sleep credit you can store. The maximum credit available is 16 points. You cannot store more than 16 points in anticipation of a long period of being awake. So even if you sleep for 10 to 12 hours after a long period of strenuous activity, you'll only get those 16 credits. That means you'll feel sleepy again after 16 hours of being awake, not after 20 or 24 hours. However, if a period of wakefulness is significantly shortened — meaning you're still in a state of sleep credit when you try to sleep again — then a good sleep is unlikely. On the debit side, the fewer points you have, the readier you are to sleep. Normally, a person will sleep when they have little or no credit left, and they'll sleep for about eight hours. That's followed by a wakeful period of about sixteen hours, during which the sleep credit gets exhausted. But here's an important point: a gradual reduction in your credit level can build up over time as what's called a 'cumulative sleep debt'. And it's critical for you as a pilot to realize that performance reduction resulting from sleep deprivation increases with altitude. That's a direct operational concern. Now let's look at how we measure sleep and its phases. Laboratory experiments have revealed a lot about the various sleep phases. Volunteers undergo several measurements while asleep. The devices used include: an Electroencephalogram, or EEG, which records the electrical activity of the brain; an Electrooculogram, or EOG, which measures eye movement within the eye socket; and an Electromyogram, or EMG, which measures muscle tension or relaxation. Sleep is classified into four stages. Stage 1 is a very light sleep — it's a transitional phase between waking and sleeping. If someone is woken during stage 1, they may claim they haven't even been asleep. In early sleep, we pass through about 10 minutes of stage 1 before moving to the deeper stage 2. Stage 2: in early sleep, we spend about 20 minutes in stage 2 before moving on to the deeper stages 3 and 4. About 50% of a normal sleep is spent in stage 2. Stages 3 and 4 are grouped together. During these stages, the brain is semiactive, emitting long slow waves as measured by EEG tracings. That's why this is commonly referred to as 'slow wave' or orthodox sleep. The eyes are stationary behind the eyelids. The muscles are relaxed. And there can be choking or crushing dreams during these stages.