Hormones, Seasons, and the Variables Nobody Warned You About (Part 11 of 12)

Hormones, Seasons, and the Variables Nobody Warned You About (Part 11 of 12)

By Ali | AliRunsOnInsulin.com | The Run Starts Before the Run Series

You’ve been following every protocol in this series. You’ve nailed your pre-run systems check, your basal adjustment timing, your fueling strategy, your post-run window. Your training log is detailed. Your gear is dialed.

And then one week your blood sugar does something completely different from what it did the week before — on the same run, at the same pace, in the same conditions. And you have no idea why.

Welcome to Part 11 of 12. This is the post for the variables that don’t show up in the standard T1D running guidance. The ones that live in your hormones, your calendar, the season, the quality of last night’s sleep. The variables nobody warned you about — because most people don’t connect them to their blood sugar until they’ve been frustrated by them for months.

The Menstrual Cycle: The Variable That Changes the Game Twice a Month

If you menstruate and have T1D, your menstrual cycle is actively participating in your blood sugar management whether you’re aware of it or not.

A typical menstrual cycle is divided into phases. During the follicular phase — roughly the first half of your cycle — hormone levels are relatively stable and most T1D women find their blood sugar management feels more predictable. Then ovulation occurs around day 14, followed by the luteal phase — the 10 to 14 days before your period starts — during which progesterone levels rise significantly.

That rise in progesterone is the key variable for T1D runners.

Research from the T1D Exercise Initiative study, which tracked menstrual cycle data alongside CGM readings in T1D women, found that total daily insulin requirements increased from the early follicular phase to the late luteal phase. Blood sugar levels trended higher in the luteal phase — the same dose of insulin that managed things well two weeks earlier is simply less effective when progesterone is elevated.

What this means practically: in the luteal phase, you may need meaningfully more insulin than usual to achieve the same blood sugar outcomes. Your running protocol from the first half of your cycle may not work the second half. This isn’t a management failure — it’s hormonal physiology.

And then your period starts. Hormone levels drop. For many T1D women, blood sugar tends to drop at the onset of menstruation as the insulin resistance that built through the luteal phase dissipates. Some experience meaningful lows in the first days of their period even when nothing else has changed.

What to do:

– Track your cycle in your training log alongside your blood sugar data — this is the only way to identify your personal pattern, because the research is consistent that individual responses vary significantly

– Expect the luteal phase to require insulin adjustments upward — your correction factor and basal rate may both need temporary increases

– Expect the first days of your period to trend lower — build in extra buffer and carry more carbs

– Time important long runs or race events with your cycle if possible — the follicular phase typically offers the most predictable management window

No two people’s patterns are the same. Some notice dramatic changes, some notice very little. The only way to know yours is to log it.

The Dawn Phenomenon: Why Morning Runs Hit Different

If you run in the morning — and many runners do — the dawn phenomenon is a variable you need to understand.

Between roughly 4am and 8am every day, the body releases a wave of hormones as part of its natural waking process. These hormones — including cortisol and growth hormone — cause the liver to release glucose into the bloodstream and create a window of mild insulin resistance. Blood glucose rises naturally during this window, in everyone, regardless of diabetes status.

For T1D runners, the dawn phenomenon means a morning fasted run is starting in a physiologically different environment than an afternoon run. Your blood sugar may already be elevated above your overnight baseline when your alarm goes off — not because of anything you ate or dosed incorrectly, but because your body did what it does every morning.

Morning runs may therefore require a different starting target and a different pre-run strategy than afternoon runs at identical intensity. Many T1D runners who log diligently notice this pattern within a few weeks: their numbers going into morning runs run higher, and their management needs to account for that.

The flip side: some T1D runners find that morning fasted runs are actually more stable than post-meal runs, because there’s no active bolus insulin in the picture. The dawn phenomenon pushes blood sugar up, the run pulls it down, and the result is a more predictable trajectory. Your log will tell you which pattern is yours.

Winter: When Cold Weather Rewrites Your Protocol

Summer running gets all the attention for blood sugar management — and we covered it in Part 8 — but winter has its own set of variables that T1D runners consistently underestimate.

Cold temperatures trigger the release of stress hormones including cortisol and adrenaline to generate body heat. Those hormones reduce insulin sensitivity and stimulate the liver to release more glucose. Studies have shown that HbA1c levels and time in target glucose range for T1D tend to be slightly higher in winter than in summer — the cold weather effect is real and documented.

For runners specifically:

Insulin absorption slows in the cold. Blood circulation is partially redirected away from the extremities to protect core temperature. This can mean insulin delivered subcutaneously absorbs more slowly, reducing its effectiveness in the way you’d normally expect. The same bolus that peaks predictably at 65 degrees may peak later and lower at 30 degrees.

Your body burns more calories in cold weather just maintaining core temperature. This can affect how quickly you deplete glycogen on a winter run, and how hungry your muscles are for glucose in the post-run recovery window.

Gear creates a carry problem. Winter running layers mean more distance between you and your CGM, your belt, and your carbs. Practice accessing your supplies while wearing your cold-weather kit before relying on it in a race or long run.

Insulin can freeze. In extreme cold, insulin exposed directly to freezing temperatures can become ineffective. If you’re running in conditions below freezing, keep any insulin you’re carrying insulated and close to your body — not in an outer pocket exposed to wind.

Sleep: The Variable You Can See in Your Numbers the Next Morning

Poor sleep raises cortisol. Cortisol increases insulin resistance. The morning after a night of disrupted or inadequate sleep, blood sugar tends to run higher and be less responsive to corrections.

When you don’t get enough sleep, your body interprets it as a stress state. This stress response leads to elevated cortisol, which elevates blood sugar by increasing insulin resistance. For T1D runners, this means a day-after-poor-sleep run often requires more careful management than a well-rested run — your insulin is less effective from the start, and you may be starting your run already fighting a higher baseline.

This is a variable worth logging. If you look back through your training data and find runs that were harder to manage than usual, check what your sleep looked like the night before. The pattern is often there.

Practically: on days after difficult nights, set your pre-run target slightly higher than usual to build in buffer, check your CGM more frequently during the run, and don’t be surprised if you need more corrections than usual.

Stress: The Invisible Blood Sugar Driver

Work deadlines. Family tension. A difficult conversation. Travel anxiety. Pre-race nerves. Anything that activates your body’s stress response releases cortisol and adrenaline — the same hormones that make race day different from training runs, now operating in your daily life.

Psychological stress raises blood sugar by making cells more resistant to insulin and stimulating the liver to release additional glucose. Even acute stress — a stressful meeting, a hard phone call — triggers this response. The effect is often subtle but meaningful, and it’s one of the harder variables to track because stress doesn’t always feel like a discrete event.

When you notice unexplained blood sugar elevation on days that otherwise look normal, check in on your stress levels. It’s not always the food, the dose, or the exercise. Sometimes it’s cortisol from the meeting you had an hour ago.

This is also why logging “stress level” as a simple low/medium/high entry in your training log pays off over time. The correlation between high-stress weeks and harder-to-manage runs is genuinely there for many T1D athletes, and you can only see it once you’ve been recording it.

Alcohol: The Delayed Variable

This belongs in the “variables nobody warned you about” category specifically because the timing of its effect is counterintuitive.

Alcohol blocks the liver from releasing its stored glucose. The blood sugar effects of alcohol can occur hours after drinking — often overnight, often during sleep — rather than immediately. This is why drinking in the evening can produce a low in the middle of the night or early morning even when your bedtime blood sugar looked completely fine.

For runners: if you had alcohol the evening before a morning run, your overnight and early morning blood sugar management may have been affected in ways that carry into how your run goes. Your liver’s ability to release glucose as backup during a developing low may be partially compromised. This is a genuine safety consideration for early morning runs after a night of drinking — not a moral judgment, just physiology worth knowing.

Fitness Changes: When Your Protocol Needs to Evolve

Here’s one that surprises runners who are months or years into a training program: as your fitness improves, your insulin needs change.

Regular endurance training increases baseline insulin sensitivity. A protocol that was calibrated for your fitness level in January may be delivering too much insulin for your fitness level in September, after months of marathon training have meaningfully increased your muscles’ efficiency at using glucose.

This is a good problem to have. But it shows up as unexplained lows during runs that used to be well-managed, or as post-run numbers that keep trending lower than your target even when you haven’t changed anything.

If you notice this pattern, your baseline management — particularly your basal rate and correction factor — may need recalibration with your care team. It’s not that your protocol broke. It’s that your body adapted, which is what training is supposed to do.

Putting It All Together

The pattern across every variable in this post is the same: your blood sugar management is not a static equation. It is a dynamic system that responds to hormonal cycles, seasons, sleep, stress, fitness, temperature, and a dozen other variables that shift across weeks, months, and years.

This is not a reason to feel overwhelmed. It is a reason to keep logging, keep observing, and keep treating every unexpected result as data rather than failure.

The runners who manage T1D most effectively over time aren’t the ones who found a perfect protocol and never deviated. They’re the ones who stayed curious about their own patterns, adjusted when the evidence called for it, and built a relationship with their body’s specific responses across all the conditions life puts them in.

Your log is building that relationship. Every entry, even the confusing ones — especially the confusing ones.

Always work with your diabetes care team when adjusting insulin for exercise. Every T1D responds differently — these protocols are starting points, not prescriptions.