The best time to train is an invention

What a new randomised study from Glasgow shows about morning vs. evening training. And why the time of day plays almost no role in training outcomes.

The 5 o'clock club has a compelling story. Getting up while it's still dark outside. Hitting the gym before the world is awake. Discipline that compounds. Sounds convincing. Only: the science does not support the narrative.

A new randomised study from Glasgow provides the clearest answer to date — and it is this: the timing of training is largely irrelevant for long-term adaptation. What matters is consistency. Not the time on the display.

What the study did

A research team led by Stuart Gray at the University of Glasgow randomised 36 healthy adults with a mean age of 30 and a BMI of approximately 28 into three groups:

  • Morning training, 6 to 10 a.m.
  • Evening training, 4 to 8 p.m.
  • Control group without structured training

Both training groups completed exactly the same programme: eight exercises, three times per week, over six weeks, at 80 percent of one-repetition maximum to the point of muscular failure. Bench press, bicep curl, lat pulldown, shoulder press, lateral raise, leg extension, leg curl, calf raise. Attendance in both groups was 100 percent.

Measurements were taken using ultrasound (muscle thickness at the vastus lateralis), one-repetition maximum, knee extension torque, and an oral glucose tolerance test for insulin sensitivity.

Ismail, Gray et al. – Glasgow 2026, Trial registration NCT05321914

The results

Both training groups showed measurable improvements. Insulin sensitivity, muscle thickness, strength. The direct comparison between morning and evening was statistically indistinguishable across all endpoints.

The study does not stand alone. Meta-analytic reviews from recent years show a consistent picture for glucose and insulin responses as well as for hypertrophy and strength gains: training timing predicts outcomes less reliably than virtually any other variable in a training plan. Progression structure, sleep quality, protein intake, consistency — all of these carry more weight than whether someone goes to the gym at 6 a.m. or 6 p.m.

The one honest nuance

There is a point at which the time of day does measurably come into play. And it deserves a clear assessment.

In cross-sectional data, most people are marginally stronger in the late afternoon. Core body temperature, testosterone-to-cortisol ratio, and muscular contraction dynamics reach a small peak between 3 and 7 p.m. Someone who always trains in the afternoon will lift slightly more in a single measurement than someone who always trains in the morning. That is real.

What follows from this is limited, however: those who train regularly in the morning adapt within a few weeks and close the gap almost entirely. The nervous system learns to perform at the trained time. Ismail et al. documented this as early as 2019 in Experimental Physiology. The afternoon peak is not a structural advantage. It is a trainable baseline.

Limits of the evidence

One important caveat: almost all available studies have been conducted with healthy younger adults. For other populations, the time of day may play a larger role.

For older individuals, the data are less clear-cut. For people with type 2 diabetes, some studies show a slight preference for afternoon or evening training with regard to glucose regulation. For those with pronounced sleep disorders, training timing can affect sleep architecture. In both directions.

The blanket recommendation that "morning is better" does not apply to these groups. A more differentiated approach is needed here, one that takes the individual data picture into account.

Where we are going with this at SLOW

This is precisely where evidence-based health optimisation differs from rules of thumb.

Viewed holistically, training time never operates in isolation. It interacts with sleep architecture, the cortisol daily profile, meal timing, work rhythm, and recovery capacity. Someone who trains at 5 a.m. and chronically sleeps too little as a result may lose more than they gain in the gym. This is not a theoretical problem. It shows up in the data.

Viewed precisely, the recommendation comes from the individual data picture. HRV trends, sleep wearable data, chronotype assessment, fasting glucose, and the cortisol daily profile allow for an answer that fits the person. Not the lifestyle aesthetic of an Instagram account.

For most healthy adults, that answer is simply this: train at the time you can realistically sustain over the long term. Consistency beats optimisation. Always.

On the SLOW platform, health professionals translate exactly these data layers into an individually tailored protocol. The 5 o'clock club myth becomes a sustainable routine. A good story becomes a measurable result. From data to results.

How do you advise clients on training time — and in which cases do you actually recommend a specific window? Write it in the comments.

Sources

Ismail AD, Gray SR et al. Resistance Exercise, Metabolism and Time of Day. University of Glasgow (2026). Trial registration NCT05321914

Ismail et al. Experimental Physiology, 104(4), 540–550 (2019). physoc.onlinelibrary.wiley.com