I've got a hilly run/walk that I do pretty much every single day. Most days I feel like it's the right level of workout for my moderate fitness level, but from time to time, I end up breathing harder and my heart beats faster than on a usual day.

What are some common causes for daily variability in what seems like purely physiological fitness level? I haven't managed to spot any obvious connections with anything going on in my routine.

1 Answer 1


Hard to really tell, but since you've tried to identify possible reasons and can't think of anything, I'll throw out what I would look at.

  • Ambient temperature or humidity might be making it more difficult on some days
  • Air pollution could vary from day to day
  • You might not be eating the same amount of food, or the same foods every day, resulting in different energy levels
  • You may have had poor sleep the night before, resulting in less energy levels
  • You may not be giving your body a chance to properly recover, so after multiple days of the same workout, your body is just saying 'enough'
  • You might be preoccupied with something, which may be adding additional stress that your body isn't adapted to handle

It might not be just any one cause, but on some days multiple things lining up to cause trouble. That could also be why you're not able to notice any trends. If any one of those things isn't really enough make it harder on you, it might look like one day you had a miserable night and had problems with your routine, and the next time you had problems you slept very well.

  • +1. I would also add sufficiently hydrated. I tend to have sluggish workouts when I've been neglecting fluid intake.
    – JohnP
    Commented Aug 23, 2012 at 3:07
  • Yes, also adding: barometric pressure, sunlight, time of day (circadian rhythm) differences, seasonal (due to day length differences), blood sugar levels or digestive drag (meal timing), other life exertions that week, stress, mild infection, body weight variations, shoe quality state, focusing more on body at some times than others and noticing things differently...and the alignment of the planets, maybe?
    – Chelonian
    Commented Sep 24, 2014 at 18:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.