eli5 – What is a persons “second wind”, and what are the physiological processes involved in getting it?


You often hear of athletes or sports persons getting a “second wind”, how is this?

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Your circadian rhythm is a 24-hour biological clock that controls how alert you feel. It fluctuates throughout the day, sending out “circadian alerting signals.” These alerting signals can either ramp up and override your sleep drive (keeping you awake) or quiet down and allow you to succumb to it (causing you to feel sleepy). Once you’ve been up for 14 or 15 hours, your internal clock has to work hard to keep you awake. (Since your sleep drive is now very high.) As a result, your circadian alerting signal is at its highest in the last few hours before bedtime. Our master clock wants cues for it to stop promoting wakefulness. It wants us to go to bed at the same time each night and follow a familiar bedtime routine.

No one knows exactly why this happens, but one theory is that there’s an evolutionary benefit: Instead of falling asleep right when the sun goes down, your second wind ensures you have enough energy to prepare a safe place to sleep (put wood on the fire, check your environment, make sure your kids are good, cover the opening of your cave).

A second wind in sports terms just refers to the athlete increasing their performance over its previous levels.

Why will vary from situation to situation, but recovering from an adrenaline dump is a likely mechanism in a lot of situations.

Basically the athlete experiences an acute surge of adrenaline early, and this lets them push themselves. When that adrenaline wears off, the body becomes lethargic and “tired”, causing performance to drop. Over time the body recovers from this, allowing the athlete to increase their performance again.

There are other similar (but less extreme) use-depletion-recovery cycles, such as ATP in muscles which are also explained by the body using a limited resource, no longer having it, and then recovering it allowing for an increase in performance over the depleted state.

The main reason for the ‘second wind’ is the body switching between energy sources. When we start excercising we burn blood sugar which is very quick to access.

Once the blood sugar depletes we get tired but will soon switch to glycogen. Glycogen is a large molecule stored in our muscles that is made of sugar units that can be broken down to restore blood sugar. When that happens we suddenly feel energized and this is the ‘second wind’.

Once glycogen is depleted we will start drawing on fat reserves. Converting to fat burning takes even more time and instead of burning sugar we are burning ketones. The biochemistry is way more complex than I’m describing but ELI5…..

I do some pretty intense cardio from time to time. If I lose track of my breathing I can end up in a situation where my blood oxygen level is sub-optimal. If I am aggressively making use of this supply, this will become apparent in minutes. Once I correct this issue my performance will return to baseline.