Marathon training and HRV: the hours between the runs

A marathon block is about sixteen to twenty weeks of being slightly tired on purpose. The skill is knowing when slightly has quietly become too much, and the run itself is the worst moment to find out, because the body lies when it is warm and motivated. The numbers that actually help are collected in the dull hours: lying in bed, asleep, standing at the kettle. Heart rate variability is the best known of them, and the research on using it to steer training is better than most fitness advice. It also has one demanding condition. It needs data almost every day.

Why HRV interests runners

Heart rate variability is the small variation in time between heartbeats. At rest, a relaxed nervous system produces more of it; stress, poor sleep, illness and accumulated fatigue tend to push it down. On its own, one number means little, because HRV differs hugely between people. Its value is personal and longitudinal: your own trend against your own baseline.

That is why it suits marathon training. The question a runner asks every morning is not whether they are fit, but whether today's planned session will build fitness or just dig the hole deeper. An HRV trend gives a second opinion on that, and a more honest one than legs that feel fine after coffee (more on what wrist HRV reveals).

What the research says

A 2024 narrative review of HRV in strength and conditioning notes that HRV-guided programming for endurance training is likely the most studied application, and that it has been associated with significant increases in maximum running velocity and VO2 max compared with predefined or block periodised plans (Addleman et al., 2024). Not a small claim.

The same review adds a caution worth taking seriously: HRV may not be a sensitive marker of overreaching or overtraining in aerobically trained athletes. It also recommends recordings of at least five minutes. A 2026 review in Sensors, on monitoring athletes through mobile devices, favours routine, near-daily measurements over isolated assessments, and suggests reading weekly averages and the coefficient of variation rather than reacting to a single morning (Sensors, 2026).

Put more bluntly: the method is only as good as the continuity of the data. Five missing days in a week leaves a weekly average built on two mornings, which is a coin toss with decimals.

When the useful data is collected

Most wrist devices compute HRV either from a short morning measurement or from the night, while you sleep. Both happen at rest, because wrist optical sensors read best when the arm is still: in a 24-hour everyday study, the share of usable signal was highest between midnight and 6 am (Merschel et al., 2022).

• Overnight: HRV and resting heart rate during sleep, the cleanest signal of the day.
• Morning: a seated or lying measurement before the day starts, if your device uses one.
• During the run: pace, distance and heart rate zones, on the smartwatch's own sport band.
• The rest of the day: background heart rate and stress trends that give context to the night.

Notice that the run is one line out of four. For recovery, the interesting data lives everywhere else (sleep tracking, in detail).

The collector's problem

Runners who love mechanical watches have a specific failure mode. The training watch is worn for the run, maybe for the night, and then comes off in the morning because the day calls for something else on the wrist. By the weekend, the baseline has holes in it, and the weekly average that the whole method relies on is quietly being built from the days on which the smartwatch happened to beat a Speedmaster in the morning negotiation at the bedside table. Holes. The method still works on paper.

There is no virtue in pretending the mechanical watch should lose. For many runners it is the reason they enjoy getting dressed at all, and a training plan that requires giving it up for four months is a plan with a quiet compliance problem (why endurance athletes end up wearing two watches).

Where the Smartlet fits

The Smartlet is a metal link bracelet that holds a mechanical watch on top of the wrist and the smartwatch underneath, in the central part of the bracelet, against the skin. The mechanical watch takes watch adapters that seat in its lug holes in place of the spring bars, and the bracelet is sized to the wrist by its links. On a working day, both are worn: the background data keeps flowing while the mechanical watch does its job.

For the run itself, the smartwatch goes back on its own sport band. No metal bracelet is made for sport, and a running watch reads best on the strap it was designed for. At night, the Clasp accessory closes the bracelet in place of the mechanical watch, so the smartwatch stays on for the overnight HRV that matters most.

Common training watches, from the Garmin Forerunner family to the Apple Watch, are listed in the smartwatch compatibility index, and the Apple Watch page covers both adapter sizes in the box. For the mechanical side, see watch brand compatibility, or mechanical watches for long-distance runners.

A training week, without gaps

The swap takes seconds once each watch carries its own pair of watch adapters (setup guide). Coaches reading this may want the running coach's version of the setup, and VO2 max and training zones goes deeper on the numbers from the run itself.

This article summarises published research on training and consumer devices. It is not medical or coaching advice. Talk to a doctor before a marathon block if you have a heart condition or symptoms.

Smartlet, Bronze Medal at Concours Lepine 2025 and shown at CES 2026, keeps the training data going on mechanical watch days.