The Shadow Sleep Cycles
Today, we’re going to learn more about sleep pressure.
When you go to sleep, you move through a series of stages. Stage 1 transitions you from being awake and drops you into stage 2. You then move into slow wave sleep, back to stage 2, and then into REM sleep. You continue to cycle through these stages multiple times throughout the night, always passing through stage 2 before entering the other stages.
Stage 2 sleep is critical for restoring mental alertness and consolidating memory. Slow wave sleep provides physical healing and helps with long-term memory formation. REM sleep promotes higher-order thinking and helps us create new neural pathways. But typically, 60% of your night is spent in stage 2 sleep.
Let’s say you go to bed at 11 PM and sleep straight through to 7 AM. If we examined your brain activity throughout the night, we’d notice the earlier cycles have a greater concentration of slow wave sleep and the later cycles have a greater concentration of REM sleep. If these sleep hours are typical for you, then your body is accustomed to this pattern and comes to expect it. That’s important, because when you’re awake, these cycles continue.
That’s because your body experiences what’s known as shadow cycles. It sounds creepy, but understanding these shadow sleep cycles is the key to unlocking the secret powers of naps.
From the moment you wake up, your body slowly builds the urge to go back to sleep. That urge is called sleep pressure, and it’s your body’s way of asking for more reparative slow wave sleep. The longer you resist the urge to sleep, the greater your sleep pressure gets, the more tired you feel, and the less functional you become. So when you wake up at 7 AM, your need for slow wave sleep is at its lowest, but as you move throughout the day, that need intensifies.
While your sleep pressure is tied to how long it has been since you last got slow wave sleep, your REM sleep ebbs and flows according to your circadian rhythm. Sleep cycles typically contain the lowest amount of REM at 9 PM, with the percentage steadily increasing until 9 AM. It then falls off again until it reaches 9 PM.
Why does this matter? Well, your ability to perform any task is subject to a similar trend with great ability at the beginning of the task, followed by a progressive decline. This is true even if you got a good night’s sleep, or conversely, a bad night’s sleep. And as we’ve read in the previous lessons, there’s strong evidence that taking a short nap can stop the decline and taking a longer nap can actually reverse it.
In the next lesson, we’ll learn more about how the timing and duration of naps can affect their effectiveness.
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