The best time of day to exercise, and where caffeine fits

Two questions come up constantly: when is the best time of day to train, and does a coffee beforehand help or just inflate the numbers? They turn out to be versions of the same question. Both change your heart rate before they change your fitness, and heart rate is what earns zone points.

The same work, a different heart rate

In "Investigation of diurnal variation in sustained exercise performance," Reilly and Garrett had seven men cycle to exhaustion at 70% of VO2 max twice — once at 08:30 and once at 17:30, in balanced order with at least three days between tests. Rectal temperature and heart rate were lower in the morning, by 0.6°C and 7 beats per minute. During the ride itself, heart rate stayed lower in the morning for the first 30 minutes. Oxygen uptake, ventilation and respiratory exchange ratio showed no time-of-day effect at all.1

That combination is the interesting part. The metabolic cost of the work was the same in the morning and the evening. Only the heart rate moved. It is a small study — seven men aged 19 to 24 — but the direction matches the underlying body temperature rhythm, which is lowest before you get up and peaks in the late afternoon.

What that costs you in points

ZonePoints scores each minute at a rate that rises in a straight line across your heart rate reserve: 1 point per minute at 50% of reserve, 6 at 100%. So every percentage point of reserve is worth 0.1 points a minute, and a few beats is not nothing.

Take a maximum of 185 and a resting rate of 60. Your reserve is 125 beats, so one beat is worth about 0.08 points a minute. Five beats sustained across a 40-minute session is roughly 16 points — against a genuinely hard session that scores around 100.

This is a real property of the score rather than a flaw in it. Zone points measure what your heart did, and at 7am your heart is doing slightly less for the same work. The practical consequence is only that a morning session and an evening session are not directly comparable. Compare mornings with mornings, and your training log will tell you the truth about whether you are getting fitter.

So when should you actually train?

The honest answer is less exciting than the question. In their review "The effect of training at a specific time of day," Hamdi Chtourou and Nizar Souissi report that time-of-day effects on anaerobic performance are well established — an early-morning low point and a late-afternoon peak — while the effect on aerobic performance is equivocal. Their more useful finding is that adaptations to training are greater at the time of day you regularly train, so training in the morning raises your morning performance towards the level you would otherwise only reach in the late afternoon.2

Reilly and Garrett's riders bear that out from the other direction: time to exhaustion ranged from 48 to 72 minutes and did not vary with time of day. Their conclusion was that continuous submaximal exercise is not necessarily disadvantaged by a morning timing.1

It is worth being careful about how strong the time-of-day claim really is. In "Circadian rhythms in sports performance — an update," Barry Drust, Jim Waterhouse, Greg Atkinson, Ben Edwards and Thomas Reilly argue that although performance is affected by time of day under normal conditions, and at least some of that variation comes from the body clock rather than from habit and daily routine, precise information on the relative importance of those two sources is lacking.3

So: the late afternoon may give you a little more for hard, short efforts, the morning takes some of that back in the score rather than in the fitness, and your body adapts to whichever you pick. The best time to train is the one you will still be training at in six months.

Does caffeine pad your score?

The intuition is obvious. Caffeine is a stimulant, so it must raise your heart rate, so it must inflate your zone points. The evidence does not support the middle step.

Mark Glaister and Conor Gissane's systematic review "Caffeine and Physiological Responses to Submaximal Exercise: A Meta-Analysis" pooled 26 double-blind, randomised crossover studies using 3–6 mg/kg of caffeine taken 30–90 minutes before sustained bouts at 60–85% of VO2 max. Caffeine had no effect on heart rate (P = .99, 207 participants), and none on oxygen consumption or respiratory exchange ratio. What it did change was blood lactate (up 0.69 mmol/L), blood glucose (up 0.42 mmol/L), ventilation (up 3.36 L/min) and ratings of perceived exertion, which fell by 0.8 points.4

Where individual trials do find a heart rate effect, it points down rather than up. In "Low doses of caffeine reduce heart rate during submaximal cycle ergometry," Steven McClaran and Thomas Wetter gave non-habitual caffeine users 1.5 and 3.0 mg/kg — about one to three cups of coffee — and found heart rate 4 to 7 beats per minute lower than placebo at all three submaximal intensities they tested, with no effect at rest or at near-maximal and maximal exercise.5

More recently, Glaister, Lythgoe and Kamath gave 16 male cyclists 5 mg/kg before a submaximal incremental test from 40 to 80% of VO2 max. They found the effect of caffeine on heart rate depended on exercise intensity (P = .019) and, at the easy end, caffeine raised heart rate variability — by 3.6, 2.6 and 0.6 ms at 40, 50 and 60% of VO2 max. Resting heart rate was not affected (P = .351). They read this as caffeine increasing the parasympathetic modulation of heart rate.6

So a pre-workout coffee does not quietly pad your weekly total. At easy intensities it may cost you a point or two, and it does not touch the resting heart rate your zones are built from.

What caffeine actually does

It changes how hard the work feels. In their meta-analysis "Effects of caffeine ingestion on rating of perceived exertion during and after exercise," Doherty and Smith pooled 21 studies and 109 effect sizes and found caffeine reduced perceived exertion during exercise by 5.6% and improved exercise performance by 11.2%. The drop in perceived exertion accounted for about 29% of the variance in that performance improvement.7

The International Society of Sports Nutrition's position stand on caffeine and exercise performance, written by Nancy Guest and colleagues, puts the effective dose at 3–6 mg/kg of body mass, most commonly taken 60 minutes before exercise, and singles out aerobic endurance as the form of exercise with the most consistent moderate-to-large benefit. Very high doses — around 9 mg/kg — bring side effects without adding benefit, and individual responses vary with genetics and with how much caffeine you normally drink.8

That is the honest route to more points. The session feels easier, so you hold a harder effort for longer, and the extra points are extra work you genuinely did.

The catch with training late

If the late afternoon is your window, the coffee that goes with it is landing closer to bedtime than you might think. In "Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed," Christopher Drake, Timothy Roehrs, John Shambroom and Thomas Roth gave 400 mg of caffeine at each of those three points and found all three significantly disturbed sleep compared with placebo. The reduction in total sleep time from the dose taken a full six hours before bed was large enough that they treat it as empirical support for stopping caffeine at least six hours before bedtime.9

A 6pm session with a pre-workout coffee is inside that window for most people. The training is fine; the timing of the caffeine is the part worth moving.

The takeaway

• Train at the time you will keep training at. Adaptation follows the hour you train in, and consistency beats chronobiology. Streaks are built on that idea.
• Compare like with like. A morning session scoring below an identical evening one is the clock, not your fitness.
• Caffeine does not inflate zone points. It earns them by making a hard effort feel manageable, which is a different thing entirely.
• Keep caffeine for the earlier sessions. Six hours before bed is still enough to cost you sleep, and sleep is where the training lands.

Want the detail on how the score is built from your heart rate? Read how training zones are calculated.

1. Reilly T, Garrett R. "Investigation of diurnal variation in sustained exercise performance." Ergonomics. 1998.
2. Chtourou H, Souissi N. "The effect of training at a specific time of day: a review." Journal of Strength and Conditioning Research. 2012.
3. Drust B, Waterhouse J, Atkinson G, Edwards B, Reilly T. "Circadian rhythms in sports performance—an update." Chronobiology International. 2005.
4. Glaister M, Gissane C. "Caffeine and Physiological Responses to Submaximal Exercise: A Meta-Analysis." International Journal of Sports Physiology and Performance. 2018.
5. McClaran SR, Wetter TJ. "Low doses of caffeine reduce heart rate during submaximal cycle ergometry." Journal of the International Society of Sports Nutrition. 2007.
6. Glaister M, Lythgoe D, Kamath S. "The Effects of Caffeine on Heart Rate and Heart Rate Variability at Rest and During Submaximal Cycling Exercise." Research Quarterly for Exercise and Sport. 2025.
7. Doherty M, Smith PM. "Effects of caffeine ingestion on rating of perceived exertion during and after exercise: a meta-analysis." Scandinavian Journal of Medicine & Science in Sports. 2005.
8. Guest NS, VanDusseldorp TA, Nelson MT, et al. "International society of sports nutrition position stand: caffeine and exercise performance." Journal of the International Society of Sports Nutrition. 2021.
9. Drake C, Roehrs T, Shambroom J, Roth T. "Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed." Journal of Clinical Sleep Medicine. 2013.