I work a lot of double shifts at my job. My normal shift is on days, then I go home for a few hours and come back at midnight to work 16 hours. The first 5 or 6 hours are a slog, and I’m constantly trying to stay awake, but then I just reach a point where I feel “normal” so to speak, even energetic at some times. What causes this feeling? How do our bodies do this?
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How energetic you feel has more to do with time of day and your daily schedule than anything else. Your body triggers certain conditions like hunger and wanting to sleep out if habit than actual need: you can go few days withour sleep if you have to; you can go weeks without eating; few days without drinking. However you feel like you have to eat, drink and sleep at certain times. This is because of circadian rhythm.
So if you push through those dips in energy that your body gets at specific times of the day, your body will normalise itself. This is what you might call 2nd wind.
It is important to remember that the biggest component for thrist, hunger and tiredness is psychological. Which is why you can psyche through them if you are aware of it. Also why some drugs that affect mood can change the way you experience these, making you feel them less or more.
If your body is actually properly tired, such as lacking nutrients, hydration or sleep, you will experience it in an entirely different manner. Your body and mind will simply stop working as well, and your blood pressure, breathing and heart rate will go wonky; along with your gut.
Your body’s energy levels go up and down throughout the day according to a particular 24 hour cycle called the circadian rhythm. Your body uses many cues to tell where it is in the 24 hour cycle, like meal times, activity, temperature, and light.
When your body thinks that it’s time to go to sleep, it activates sleep mode by releasing lots of melatonin to make you sleepy, so that’s how you feel for a few hours. After a while your body metabolises all of the melatonin, and since you’re working (maybe you’re physically active too), and because of all the light you see, your body soon activates awake mode by releasing lots of cortisol to wake you up again. When you get your second wind, it’s your body basically saying “I’m going to stop trying to sleep now and instead I’m going to wake up and be ready for the day”.
Sleep has two systems. The first is sleep-wake drive. It makes to feel sleepy the longer you have been awake and less sleepy the longer you have been asleep.
The second is circadian rhythm. It will keep you awake when the sleep drive would have you sleep and keeps you asleep when you would be awake.
There can be different factors for a second wind. If it occurs in the morning around 6-8am that is probably because the circadian rhythm usually produces cortisol at that time which helps wake you up. If that’s not a normal time you wake up than that cortisol production might be different.
At night your sleep drive and circadian rhythm work together to get you to sleep. In what is called a sleep gate. The sleep drive is depleted and circadian rhythm produces melatonin. If you fail to fall asleep before the peak in melatonin than you might experience a second wind because the production slows down.
Third there is a forbidden sleep of wake maintenance zone. These are periods during the day where the need for sleep is low. One time is 1-3 hours before typical bedtime. It is caused by a production dopamine in proportion to how long you have been awake. This is a counterbalance to adenosine which the sleep drive uses to make you sleepy.
I recently watched a GDC talk from someone describing crunch work culture at video game companies that said that the “second wind” is a myth and what you are actually experiencing is the part of your mind that tells you that you’re tired is falling asleep. The period of the second wind can feel like you are more energetic but in reality you are just as tired/out of energy you just aren’t trying to conserve what little you have left anymore because your brain is too tired to self-manage.
Lot of psychology/sleep/biorhythm stuff here. Not discounting any of that, but in athletics, a ‘second wind’ is essentially when your body changes fuel sources. In general, your muscles store a small amount of glucose, and this is what they use to do work. Consuming this glucose produces water, CO2, and small amounts of lactic acid (the stuff that makes your muscles sore after a workout). When the glucose is exhausted, generally your body will start to move other sugars into your muscle cells to replace it. Roughly the same chemical reaction occurs. Finally, when those readily available sugars in your bloodstream are exhausted, your body will turn to stored energy, typically lipids (fat). Lipids are very high in energy concentration, but are not as easily broken down and produce more lactic acid. So if you are going for a run, and you can easily do a mile or two, but then get tired and push through, and then get your ‘second wind’ on mile 3 or 4, this is because you are now burning a more concentrated and powerful fuel. And also why you will be more sore the next day.
I’ve left A LOT out here, but this is ELI5. In relation to OP’s question, sleep cycle and what is now your ‘new’ circadian rhythm probably have a lot to do with it, especially if you are inside and don’t really see the sun. However, your body still uses the same ‘fuel.’ If you wake up and go to work for 5-6 hours, you probably have some stored glucose that you burn through fast and don’t really replace. When you go home and rest, you both replace that glucose and essentially ‘power up’ for the next 16 hour round. Because you have already burned off the ‘days’ worth of fast sugars, you are operating on the higher octane fuel for the second half of your shift. Thus, you feel more energetic and smooth for the second part.
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