Cot death, known medically as sudden infant death syndrome (SIDS), is the unexplained death of a baby under one year old, typically during sleep. Most cases occur between 1 and 4 months of age, and over 90% happen before a baby reaches 6 months. Despite decades of research, no single cause has been identified, but a combination of biological vulnerabilities and environmental factors is involved. The good news: safe sleep practices have dramatically reduced cot death rates since the 1990s.
How Cot Death Differs From Other Infant Deaths
Cot death falls under a broader category called sudden unexpected infant death (SUID), which also includes accidental suffocation in a sleeping environment and deaths from other unknown causes. What makes SIDS distinct is that it remains unexplained even after a thorough investigation, including a complete autopsy, examination of the death scene, and review of the infant’s medical history. When all known causes have been ruled out, the death is classified as SIDS.
In the United States, about 3,700 sudden unexpected infant deaths occurred in 2022. Of those, 1,529 were specifically classified as SIDS.
Why It Happens: The Triple Risk Model
Researchers use what’s called the Triple Risk Model to explain cot death. It proposes that SIDS occurs when three factors overlap at the same time: the baby is in a critical developmental period, the baby has an underlying vulnerability, and the baby encounters an external stressor.
The critical developmental period covers roughly the first six months of life, when a baby’s body is rapidly changing how it regulates sleep, breathing, heart rate, blood pressure, and temperature. These systems are inherently unstable during this window. The underlying vulnerability is some defect or abnormality the baby is born with, often involving the parts of the brain that control breathing and arousal from sleep. The external stressor is something in the environment, like sleeping face down, loose bedding, overheating, or exposure to secondhand smoke. A healthy baby can typically overcome these challenges. A vulnerable baby may not.
This model explains why cot death doesn’t have a single preventable cause. It also explains why reducing environmental stressors, even when you can’t detect an underlying vulnerability, significantly lowers the risk.
What’s Happening in the Brain
The most consistent finding in SIDS research involves abnormalities in the brainstem, the part of the brain that handles automatic functions like breathing and waking up. Specifically, researchers have found defects in cells that produce serotonin, a chemical messenger critical for regulating breathing, heart rate, and the ability to wake up in response to danger.
In babies who died of SIDS, studies have found reduced activity in serotonin receptors across multiple areas of the brainstem. These same babies had more serotonin-producing cells than normal, but the cells appeared immature and didn’t function properly. This matters because serotonin neurons are responsible for detecting rising carbon dioxide levels during sleep. When a healthy baby’s face gets buried in bedding, rising CO2 triggers the brain to wake the baby or turn their head. In a baby with defective serotonin signaling, that alarm system may fail.
Under normal conditions, serotonin neurons triple their firing rate when CO2 rises. But during early postnatal development, this response is naturally blunted, meaning all young babies have a window of vulnerability. Babies born with serotonin system defects face a compounded risk during this already fragile period.
Genetic Vulnerabilities
About one in five babies who die of SIDS carries a mutation in genes related to the heart’s electrical system. These mutations affect ion channels, the tiny gates that control how electrical signals move through heart muscle. When these channels malfunction, they can cause dangerous heart rhythms.
The most commonly affected gene is one that controls the heart’s sodium channel. Mutations here can cause the heart to take too long to reset between beats, a condition related to long QT syndrome. Other mutations affect the heart’s ability to handle calcium signaling during rest, making the heart prone to erratic rhythms during sleep. These genetic defects are present from birth but may never cause problems unless combined with the other risk factors in the Triple Risk Model, like sleeping face down or overheating.
When Risk Is Highest
Cot death can happen at any point during the first year, but the risk is heavily concentrated in the early months. Seventy-two percent of SIDS deaths occur between months 1 and 4. The risk drops significantly after 6 months, which aligns with the period when a baby’s brainstem systems for breathing, arousal, and temperature regulation become more stable. Premature and low birth weight babies face higher risk because their regulatory systems are even less mature.
Overheating and Temperature Stress
Overheating is one of the most underappreciated risk factors for cot death. When a baby gets too warm during sleep, it places extra demand on the body’s temperature regulation system. In a vulnerable infant, this thermal stress can suppress the drive to breathe, impair the reflex that keeps the airway open, and blunt the ability to wake up.
Bedroom heating, overwrapping, and heavy bedding all increase risk. Well-ventilated bedrooms and even the use of a fan have been associated with lower risk. Signs your baby may be too warm include sweating, damp hair, flushed cheeks, or a hot chest. A room that feels comfortable to a lightly clothed adult is generally appropriate for a baby dressed in one additional layer.
How Pacifiers May Protect
Offering a pacifier at sleep time is associated with reduced SIDS risk, though the exact reason isn’t fully settled. Several mechanisms likely contribute. Sucking on a pacifier helps keep the tongue forward, which maintains an open airway during sleep. The bulky handle of the pacifier may also keep bedding slightly away from the baby’s nose and mouth, reducing the chance of accidental smothering. Pacifier use appears to lower the arousal threshold, meaning the baby wakes more easily in response to problems like a drop in oxygen or an abnormal heart rhythm. There’s also evidence that non-nutritive sucking helps clear stomach acid that refluxes during sleep, reducing another potential trigger.
Safe Sleep Practices That Reduce Risk
The single most important step is placing your baby on their back for every sleep, including naps. Babies who sleep on their backs are significantly less likely to die of SIDS than those placed on their sides or stomachs. Beyond positioning:
- Use a firm, flat mattress in a safety-approved crib or bassinet, covered only by a fitted sheet. Inclined surfaces, swings, and car seats are not safe for unsupervised sleep.
- Keep the sleep area bare. No blankets, pillows, bumper pads, stuffed animals, or loose bedding. If you’re worried about warmth, use a wearable sleep sack.
- Avoid weighted sleep products. Weighted swaddles, blankets, and sleep sacks are not considered safe for infants.
- Share a room, not a bed. Having your baby sleep in your room (in their own crib or bassinet) for at least the first 6 months can reduce SIDS risk by as much as 50%. Bed sharing increases risk.
- Don’t let your baby overheat. Avoid heavy clothing and keep the room comfortably cool.
A Possible Biomarker Identified at Birth
A 2022 study published in eBioMedicine found that babies who later died of SIDS had measurably lower levels of an enzyme involved in the nervous system’s signaling pathways, detectable from a blood sample taken just 2 to 3 days after birth. SIDS babies had an average enzyme activity of 5.6 units per milligram compared to 7.7 in surviving babies. Babies who died of other causes showed no such difference from controls.
This suggests that at least some SIDS babies are born with a detectable deficit in their autonomic nervous system, the system responsible for unconscious functions like breathing and heart rate during sleep. While this finding hasn’t yet led to a screening test, it’s the first biochemical marker that can distinguish SIDS-vulnerable babies before death occurs, reinforcing the idea that cot death involves a real, biological vulnerability rather than being purely random.