Baby sleep glossary: 40 common terms explained simply

There are 40 terms here, in alphabetical order, each explained in one to three sentences. We describe methods; we don’t recommend them. Every safety and research statement comes from the AAP, CDC, NIH and peer-reviewed sources listed at the end. Each term can be linked to directly, for example /guides/baby-sleep-glossary/#wake-window.

Learning the vocabulary is a start. Figuring out which terms actually explain your own baby’s nights is the harder part, and Actually Sleeping does that with each family: we write a sleep plan for that specific baby, coach you bootcamp-style before the first night, and check in 1:1 every day.

Go straight to a term

Active sleep · Bed sharing · Bedtime fading · Camping out · Catnap · Chair method · Circadian rhythm · Cluster feeding · Contact nap · Corrected age · Dream feed · Drowsy but awake · Early rising · Extinction · False start · Graduated extinction · Melatonin · Nap transition · Night weaning · Overtired · Quiet time · REM sleep · Room sharing · Safe sleep · Self-settling · SIDS · Sleep association · Sleep cycle · Sleep debt · Sleep pressure · Sleep regression · Sleep sack · Split night · SUID · Swaddle · TOG · Undertired · Wake window · White noise · Witching hour

The glossary

Active sleep: What REM sleep looks like in a newborn: twitches and grimaces, uneven breathing, and eyes that may flicker. Newborns enter sleep through this stage instead of through quiet sleep, which helps explain why they wake so easily.
Bed sharing: When a baby sleeps on the same bed, sofa or other surface as an adult or another child. The AAP advises against bed sharing in all circumstances, twins and other multiples included.
Bedtime fading: A method where bedtime is shifted later for a while, to the point where the child reliably falls asleep fast, and then brought earlier in small steps. A 2016 randomized trial found it reduced how long infants took to fall asleep. Read more in bedtime fading.
Camping out: A method with the parent present: over a series of nights, the parent stays in the room as the child falls asleep, moving steadily farther away and finally out. People often treat it as the same thing as the chair method.
Catnap: A brief nap, frequently a single sleep cycle or shorter. Parents also use the word for the short late-afternoon nap that many younger babies take ahead of bedtime.
Chair method: At bedtime the parent sits in a chair next to the crib, then moves the chair slightly farther away every few nights until it leaves the room. Read the chair method.
Circadian rhythm: The internal clock, running on roughly a 24-hour cycle, that times hormones, alertness and sleepiness. It develops in babies at about 2 to 3 months, which is why newborns sleep in scattered bits across the day and night.
Cluster feeding: A common term for a period of very frequent, closely spaced feeds, typical of newborns and young infants and often happening in the evening.
Contact nap: A nap taken while an adult holds or wears the baby. The AAP says that if you might doze off, move the baby to their own sleep space first; it notes that infants sleeping with someone on a couch, soft armchair or cushion face up to 67 times the risk of sleep-related death. Read how to stop contact napping.
Corrected age: For a baby born early, the age since birth minus the number of weeks they arrived preterm (40 minus gestational age at birth). The AAP suggests using it through the first 2 years when judging when developmental milestones should appear.
Dream feed: A feed offered to a sleeping baby without fully waking them, typically late in the evening before the parent turns in. Some families try it to push the baby’s longest stretch of sleep later. Read does a dream feed work.
Drowsy but awake: Laying a baby in the crib when they are sleepy and relaxed but not yet asleep, so the last step of drifting off happens there. It is often suggested as a way to help a baby learn to fall asleep on their own.
Early rising: Starting the day sooner than the family would like and not falling back asleep. Parents define it as a problem; it isn’t a medical diagnosis. Read early morning waking.
Extinction: The parent puts the child down at a set bedtime and doesn’t go back in before morning unless there is a safety issue or illness. Researchers refer to it as unmodified extinction, and a 2006 AASM review rated its research support as strong. Read extinction explained.
False start: A waking shortly after bedtime, frequently 30–45 minutes in, as though the first sleep cycle were a nap. Read false starts at bedtime.
Graduated extinction: The parent leaves, then comes back for short checks at intervals that get longer over time. A 2016 randomized trial found it cut night awakenings and time to fall asleep, and at 12-month follow-up showed no differences in attachment or in emotional or behavioral problems. Read graduated extinction explained.
Melatonin: A hormone released in darkness that tells the body it is night; babies show a day-night melatonin rhythm by about 3 months. In the U.S. it is also sold as a supplement, which the FDA does not regulate the way it regulates medicines, and the AAP advises using it only after a conversation with your pediatrician.
Nap transition: The weeks in which a child goes from one number of naps to fewer, such as 3 naps to 2 or 2 to 1. During the switch, days are often a mix of both. Read nap transitions.
Night weaning: Reducing or ending nighttime feeds, gradually or completely. At Actually Sleeping, your custom plan sets how night feeds are handled, based on your baby’s age. Read night weaning.
Overtired: What parents call a baby who has stayed awake too long for their age and grows harder, rather than easier, to settle. It is a practical description, not a medical diagnosis. Read overtired baby.
Quiet time: A rest period spent in the child’s room, typically once a toddler or preschooler has stopped napping, during which the child plays quietly rather than sleeps.
REM sleep: Rapid eye movement sleep, the lighter stage associated with dreaming. A full non-REM and REM cycle lasts about 50 minutes in infants, versus about 90 minutes in adults.
Room sharing: The baby sleeps in the parents’ bedroom on a separate, safe surface of their own. The AAP recommends this for at least the first 6 months and says it can lower SIDS risk by as much as 50%.
Safe sleep: The practices that lower the risk of sleep-related infant death: on the back for every sleep, a firm and flat surface, nothing soft in the sleep space, and room sharing without bed sharing. According to the AAP, no surface inclined more than 10 degrees is safe for sleep. Read safe sleep and sleep training.
Self-settling: Getting to sleep, or back to sleep after waking, with no help from an adult. It also goes by self-soothing or independent sleep.
SIDS: Sudden infant death syndrome: the sudden death of a baby under 1 year old for which no cause is found even after a full investigation. It is one kind of SUID.
Sleep association: Anything a child connects with falling asleep, like feeding, rocking, a pacifier or having a parent nearby. It matters most when the child needs that same thing again to get back to sleep after ordinary night wakings. Read feed-to-sleep association.
Sleep cycle: A single round of non-REM and REM sleep. In infants a cycle runs about 50 minutes, and a short partial waking between cycles is normal. Read how infant sleep cycles develop.
Sleep debt: The accumulated gap between how much sleep someone needs and how much they actually get. Parents apply it loosely to a child who has had several short nights or naps in a row.
Sleep pressure: The urge to sleep, which grows the longer a person stays awake and fades during sleep. Wake windows rest on this idea: with enough pressure a baby falls asleep readily, and with too little, settling drags on.
Sleep regression: A popular name for a period, often near 4, 8–10, 12 or 18 months, when a child who had been sleeping well begins waking more or resisting naps. It is not a medical diagnosis. Read the 4-month regression.
Sleep sack: A blanket the baby wears. The AAP favors infant sleep clothing over loose blankets, says non-swaddling sacks that allow free movement can be used for as long as you like, and advises against weighted blankets, sleepers, swaddles or any other weighted objects on or near your baby.
Split night: An extended period awake in the middle of the night, frequently 1–3 hours, during which the child is calm or playful instead of distressed. Read split nights.
SUID: Sudden unexpected infant death: the sudden and unexpected death of a baby under 1 year old. This broad category takes in SIDS, accidental suffocation in a sleeping environment, and deaths whose cause is unknown.
Swaddle: Wrapping a baby firmly, arms in, in a thin blanket or a swaddle product. The AAP says to stop once a baby shows signs of attempting to roll, to give the hips room to move, and to avoid overheating, and it states there is no evidence supporting swaddling as a way to lower SIDS risk. Read swaddle transition.
TOG: Thermal overall grade, a measure of how much warmth a fabric retains. A higher tog value means more insulation, and some sleep sacks carry this rating.
Undertired: What parents call a child put down before sleep pressure has built up enough, who then plays, chatters or takes ages to drift off. As with overtired, it is a practical description rather than a diagnosis.
Wake window: The time a baby spends awake between one sleep and the next. Wake window ranges by age are practical rules of thumb, not research findings. Read the wake window calculator.
White noise: A constant, even sound that covers household noise while a child sleeps. A 2014 study found that every one of the 14 infant sleep machines tested could go above 50 decibels at 30 cm, so both volume and distance from the crib matter.
Witching hour: What parents call a predictable stretch of evening fussiness in young babies. The AAP’s parent site notes that fussy periods tend to fall between 6 p.m. and midnight.

Sources for the figures

The numbers used in these definitions (cycle length, room-sharing guidance, the 10-degree incline limit, sound levels) appear on the baby sleep statistics page or in the references below, and each was checked against its source page.

Turning vocabulary into a plan

We define the methods above purely for reference. Actually Sleeping doesn’t use cry it out (extinction), and we don’t ask families to wait it out. We write a plan around your baby’s age, feeds, temperament and current sleep associations, then coach naps until they are solid. Your custom plan sets how night feeds are handled, based on your baby’s age. Parents talk about their results in the client video reviews.

If more of these terms describe your baby than you’d like, book the free consultation.

Want your baby sleeping like this?

With Actually Sleeping you get 1:1 coaching: Dr. Sam writes a sleep plan for your baby, we coach you bootcamp-style before the first night, and we check in daily until it is working. We have coached 700+ families, collected 129 video reviews, and put our guarantee in writing: 8–12 hours through the night within 14 days, or a full refund.

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Common questions

How do SIDS and SUID differ?

SUID, or sudden unexpected infant death, is the broad term covering any sudden, unexpected death of a baby younger than 1. SIDS is one kind of SUID: a death that still has no known cause after a full investigation. Another kind is accidental suffocation in a sleeping environment.

Does a sleep regression count as a medical condition?

No. The phrase is a popular label for a stretch when a child who had been sleeping well begins waking more or fighting naps, often alongside developmental changes. Call your pediatrician if changes in sleep come with fever, pain or other symptoms.

How does extinction differ from graduated extinction?

With extinction, the parent doesn’t go back in after bedtime other than for safety or illness. With graduated extinction, the parent goes back for short checks spaced at intervals that grow longer over time. Research has studied both behavioral methods; a child’s sleep can improve without either, and Actually Sleeping uses neither one.

Do wake windows come from research?

No. Wake window ranges by age are conventions from practice. What research recommends is total sleep per 24 hours: the AASM sets 12–16 hours for 4–12 months and 11–14 hours for 1–2 years, and gives no figure under 4 months.