Last week I was in a terrible mood the entire week, and at a baseline I'm quite a happy person — a very happy person. Not only was this super uncharacteristic of me, but by the end of the week I was annoyed and tired of being cranky.

I tried to find some answers. First: I'm just not getting to work enough, I'm stressed about work. Maybe that's why I'm grumpy. Then: maybe it's that I'm not happy with my body right now. I'm still dealing with diastasis recti and, in general, decreased muscle strength.

Finally, I realized it had been about a month or less since I had stopped breastfeeding — and I realized it was most likely that I was experiencing some intense hormonal changes that were just wreaking havoc on everything else (breaking out, having super dry skin, not sleeping well, etc.).

So I dug into the research on how dramatic the hormone shift is just after weaning. I’m happy to report that it’s not just in our heads.

When you stop breastfeeding, prolactin drops within 24 hours. That’s fine. Expected. No more milk is needed.

But those oxytocin pulses, the happy, love hormone, which was triggered by every feeding, stop entirely and dramatically 😢. Cortisol, which was suppressed during lactation, starts climbing back toward its pre-pregnancy baseline 🫠.

At the same time, your body restarts its normal hormonal cycle. Ovulation (and your period + cramps) typically resumes within 14–30 days. Your system is going from a carefully maintained lactation state to full hormonal reactivation, and doing that, in a matter of days. It’s really fast.

This transition is well-documented. What it feels like, and how long it lasts, is where the science still shows gaps. (Though perhaps, we don’t need the science to tell us that, individually, it shows up in our mood and energy levels.)

The mood piece has the strongest evidence. Case reports going back to the late 1980s link major depressive episodes directly to the timing of weaning.

Observational studies consistently show that women who stop breastfeeding earlier have higher rates of postpartum depression. This runs in both directions, however, since depression also causes women to stop breastfeeding sooner.

The most compelling biological explanation involves a brain chemical called allopregnanolone: a hormone derived from progesterone that calms the brain by acting on GABA receptors (the same system targeted by anti-anxiety medications).

During lactation, your hormonal environment keeps this system relatively stable. When cycles resume and progesterone starts fluctuating again, women who are sensitive to these shifts can experience anxiety, low mood, and emotional dysregulation, not at delivery, but weeks later, timed to weaning and first period return.

This is the same pathway targeted by brexanolone, the first FDA-approved postpartum depression drug.

The biology is makes sense, but we haven’t done a controlled study that randomizes women to stop vs. continue breastfeeding and tracks mood outcomes directly.

Fatigue and appetite changes specifically at weaning have not been studied in humans in any controlled way. Most of what we know comes from animal models (sows, seals, piglets) where the post-weaning period involves clear metabolic reorganization: insulin rises, growth hormone increases, and animals that built up an energy deficit during lactation take longer to recover.

Breastfeeding burns roughly 300–500 extra calories per day. When that demand disappears, your body's appetite signals don't necessarily recalibrate overnight.

Residual fatigue from months of lactation, disrupted sleep, and hormonal flux is biologically plausible. It just hasn't been measured directly in weaning women.

While we have insights into the mechanisms at play and we can rationalize the cascade of biological happenings in the weeks after weaning, we ultimately don’t have clear evidence for female humans. Still, we do what we can.

Here are a few things you can do to minimize the impacts on mood and energy during the weaning period:

🧠 Track your mood against your cycle after weaning.
Log mood daily for 6–8 weeks post-weaning. If your lowest days cluster around your first period return, that's likely the allopregnanolone.

🐟 Add omega-3s daily during the weaning window.
Take 1–2g of combined EPA/DHA per day. Omega-3s are associated with better mood outcomes in postpartum women broadly, and the anti-inflammatory effect is most relevant when your hormonal system is in flux.

🥗 Don't let your calorie intake drop off sharply when you stop.
Lactation burns 300–500 kcal/day. Your hunger cues may not adjust immediately. Keep eating enough (especially protein) for at least 4–6 weeks after weaning to support energy and muscle recovery.

💊 Check your B12, magnesium, and vitamin D status.
These three micronutrients are consistently depleted in postpartum women and are directly linked to fatigue and mood regulation. If you haven't had labs since delivery, now is a good time.