Yes, breastfeeding can make you tired, and it’s not just from the night wakings. Producing breast milk burns an extra 330 to 400 calories per day, triggers hormones that physically relax your body, and increases your need for fluids and nutrients. The fatigue most breastfeeding parents feel comes from several of these factors stacking on top of each other at once.
Your Body Is Running a Small Factory
Making breast milk is metabolically expensive. The CDC recommends breastfeeding mothers consume 330 to 400 extra calories per day compared to what they ate before pregnancy, just to keep up with milk production. That’s roughly equivalent to an extra meal. If you’re not eating enough to cover that demand, your body pulls from its own energy reserves, leaving you feeling drained even on days when you managed decent sleep.
This caloric cost is constant. Unlike exercise, which you do in bursts, milk production runs around the clock. Your body is synthesizing fats, proteins, and sugars into milk whether you’re awake or asleep, which means the energy drain doesn’t pause when you rest.
Hormones That Make You Drowsy
Two key hormones surge every time you nurse, and both have a sedating effect. Oxytocin, released during letdown, reduces anxiety and dampens your body’s stress response. It also promotes a calm, still posture, which researchers believe evolved to keep mothers motionless while their infant feeds. That wave of relaxation many parents feel the moment a baby latches isn’t imagined. It’s a neurological signal encouraging you to stay quiet and nurture.
Prolactin, the hormone that drives milk production, has its own calming properties. It dials down your body’s stress-hormone system, lowering reactivity to emotional and physical stressors. The combination of these two hormones during a feeding session can feel a lot like sleepiness, especially if you’re already running on limited rest. Some parents describe it as a heaviness that settles over them within minutes of starting a feed.
Night Wakings and Sleep Quality
Breastfed babies under six months wake more often at night than formula-fed babies, though total sleep over 24 hours is generally similar between the two groups. After six months, the gap widens: most studies find breastfed infants sleep less at night and less overall compared to formula-fed infants. This means the parent doing overnight feeds accumulates a sleep debt that compounds over months.
It’s not just total hours that matter. Fragmented sleep is less restorative than uninterrupted sleep, even when the minutes add up to the same number. Waking every two to three hours to nurse prevents you from completing full sleep cycles, so you can sleep “enough” hours and still feel exhausted the next day. This is one of the biggest contributors to breastfeeding-related fatigue, and it’s the hardest one to solve while exclusively nursing a young infant.
Fluid and Nutrient Gaps
Breastfeeding mothers need roughly 16 cups of fluid per day from food, beverages, and water combined. That’s significantly more than the standard recommendation for non-lactating adults. Even mild dehydration can cause fatigue, brain fog, and headaches, and it’s easy to fall short when you’re busy caring for a newborn and forgetting to refill your water bottle.
Nutrient deficiencies also play a role. Vitamin B12 deficiency is a particular concern, especially for vegetarian or vegan mothers, since B12 comes almost exclusively from animal products. Low B12 in a breastfeeding parent can cause fatigue, nerve tingling, and in severe cases, neurological damage. Iron, iodine, vitamin D, calcium, and zinc are other nutrients that can run low during lactation. If your tiredness feels disproportionate to your sleep loss, a nutritional gap is worth investigating.
Postpartum Anemia
About 22% of women have anemia in the first 48 hours after giving birth, and nearly half have hemoglobin levels below normal ranges in the early postpartum period. Blood loss during delivery is the primary cause, and anemia doesn’t always resolve on its own in the weeks that follow. If you lost a significant amount of blood during birth, had a long labor, or went into delivery already low in iron, your risk is higher.
Anemia makes everything harder. It reduces the oxygen-carrying capacity of your blood, so normal activities feel more effortful. The fatigue from anemia is distinct from sleep deprivation: it’s a bone-deep exhaustion that doesn’t improve much with rest. A simple blood test can check your hemoglobin and ferritin levels. Many guidelines recommend screening at four to eight weeks postpartum, but if you’re struggling with severe fatigue, it’s reasonable to ask sooner.
Thyroid Problems After Delivery
Postpartum thyroiditis affects 5% to 10% of people after birth and is an often-overlooked cause of crushing fatigue. The condition typically moves through two phases. An initial overactive thyroid phase can cause fatigue and anxiety, followed by an underactive phase that brings low energy, weight gain, cold intolerance, and depression. Because these symptoms overlap so heavily with “normal” new-parent exhaustion, thyroid problems frequently go undiagnosed for months.
The underactive phase is particularly easy to dismiss. Feeling cold, sluggish, and gaining weight while breastfeeding can seem like just another part of postpartum life. A thyroid panel is a straightforward blood test, and it’s worth requesting if your fatigue is severe, worsening over time, or accompanied by unusual weight changes or mood shifts.
Practical Ways to Manage the Fatigue
You can’t eliminate the biological cost of breastfeeding, but you can stop it from compounding unnecessarily. Eating enough is the simplest intervention most people overlook. Those extra 330 to 400 calories don’t need to come from elaborate meals. Calorie-dense snacks you can eat one-handed while nursing (nuts, cheese, granola bars, avocado toast) close the gap without requiring extra prep time.
Keep water within arm’s reach of every spot where you regularly feed. Many parents find they’re intensely thirsty during letdown, which is your body signaling a real need. A large water bottle at your nursing station is a small change that makes a measurable difference.
If you’re following a vegetarian or vegan diet, a B12 supplement is essential during lactation, not optional. For everyone, a postnatal vitamin that includes iron, iodine, and vitamin D provides a safety net against the most common deficiencies that drive fatigue. If you’ve been exhausted for weeks and sleep improvements aren’t helping, asking for bloodwork to check your iron stores and thyroid function can rule out or catch medical causes that have straightforward treatments.