You've had the workout that felt effortless, and you've had the identical workout — same routine, same weights, same distance — that felt like wading through wet sand for no clear reason. Most people assume that's random. It isn't. A meaningful piece of it comes down to light, and when your body received it.
The mechanism, briefly
Your body temperature, alertness, and physical performance all follow a daily rhythm governed largely by your circadian clock — and that clock is set primarily by light exposure, especially light received in the morning. Core body temperature naturally rises across the day and tends to peak in the late afternoon to early evening for most people with a well-anchored circadian rhythm, which is part of why many people report their best raw physical performance in that window.
But here's the part that actually matters day-to-day: that afternoon peak depends on your clock being properly set in the first place — and the clock gets set by morning light. Skip consistent morning light exposure, and the whole rhythm can drift, flatten, or shift later, which is a big part of why some days you simply don't have the "afternoon peak" people talk about, no matter how disciplined your training schedule is.
So the honest answer to "when's the best time to work out" isn't just a clock-time. It's: get real light exposure earlier in the day, consistently, and your body will build toward its own natural performance window later — reliably, rather than as a matter of luck.
Why this gets missed constantly
Fitness advice is saturated with timing hacks — fasted cardio, evening lifts, morning cardio for "metabolism." Almost none of it addresses the actual upstream variable: whether your circadian rhythm is anchored well enough for any of those windows to mean something consistent. Training at 5pm because "that's when performance peaks" doesn't help much if your daily light exposure has been so inconsistent that your body doesn't have a clear peak to begin with.
The bigger pattern
This is a small, practical example of a much larger idea: performance, energy, and recovery aren't things you can force with better timing alone. They're downstream of whether your body is getting the environmental signals it needs to run its own rhythms properly. Light is the biggest one.
A consistent morning light session — as part of a daily Mitolux practice — is one of the simplest ways to anchor that rhythm before you ever get to the gym. Not a pre-workout. A way of setting the clock the rest of your day, and your training, runs on.