A lullaby is a soothing piece of music, usually played or sung to young and old children. The purposes of lullabies vary. In some societies they are used to pass down cultural knowledge or tradition. In addition, lullabies are often used for the developing of communication skills, indication of emotional intent, maintenance of infants' undivided attention, modulation of infants' arousal, and regulation of behavior. Perhaps one of the most important uses of lullabies is as a sleep aid for infants. As a result, the music is often simple and repetitive. Lullabies can be found in many countries, and have existed since ancient times.

Lullabies tend to share exaggerated melodic tendencies, including simple pitch contours, large pitch ranges, and generally higher pitch. These qualities are exaggerated to clarify and convey heightened emotions, usually of love or affection. When there is harmony, infants almost always prefer consonant intervals over dissonant intervals. Furthermore, if there is a sequence of dissonant intervals in a song, an infant will usually lose interest and it becomes very difficult to regain its attention. To reflect this, most lullabies contain primarily consonant intervals. Tonally, most lullabies are simple, often merely alternating tonic and dominant harmonies.

In addition to pitch tendencies, lullabies share several structural similarities. The most frequent tendencies are intermittent repetitions and long pauses between sections. This dilutes the rate of material and appeals to infants' slower capacity for processing music.

Rhythmically, there are shared patterns. Lullabies are usually in triple meter or 6/8 time, giving them a "characteristic swinging or rocking motion." This mimics the movement a baby experiences in the womb as a mother moves. In addition, infants' preference for rhythm shares a strong connection with what they hear when they are bounced, and even their own body movements.  The tempos of lullabies tend to be generally slow, and the utterances are short. Again, this aids in the infant's processing of the song.

Lullabies almost never have instrumental accompaniments. Infants have shown a strong preference for unaccompanied lullabies over accompanied lullabies. Again, this appeals to infants' more limited ability to process information.

Lullabies are often used for their soothing nature, even for non-infants. One study found lullabies to be the most successful type of music or sound for relieving stress and improving the overall psychological health of pregnant women.

These characteristics tend to be consistent across cultures. It was found that adults of various cultural backgrounds could recognize and identify lullabies without knowing the cultural context of the song. Infants have shown a strong preferences for songs with these qualities.

Cross-cultural prevalence
Lullabies are often used to pass down or strengthen cultural roles and practices. In an observation of the setting of lullabies in Albanian culture, lullabies tended to be paired with the rocking of the child in a cradle. This is reflected in the swinging rhythmicity of the music. In addition to serving as a cultural symbol of the infant's familial status, the cradle's presence during the singing of lullabies helps the infant associate lullabies with falling asleep and waking up.

Therapeutic value
Studies conducted by Dr. Jeffery Perlman, chief of newborn medicine at NewYork–Presbyterian Hospital's Komansky Center for Children's Health, find that gentle music therapy not only slows down the heart rate of prematurely delivered infants but also helps them feed and sleep better. This helps them gain weight and speeds their recovery. A study published in May 2013 in the Journal of the American Academy of Pediatrics under the aegis of the Beth Israel Medical Center in New York City found that the type of music matters. Therapeutically designed "live" music – and lullabies sung in person – can influence cardiac and respiratory function. Another study published in February 2011 in Arts in Psychotherapy by Jayne M. Standley of the National Institute for Infant and Child Medical Music Therapy at Florida State University suggests that babies who receive this kind of therapy leave the hospital sooner.

Additional research by Jayne M. Standley has demonstrated that the physiological responses of prematurely delivered infants undergoing intensive care can be regulated by listening to gentle lullabies through headphones. In addition to slowing heart and respiration rates, lullabies have been associated with increased oxygen saturation levels and the possible prevention of potentially life-threatening episodes of apnea and bradycardia. Gentle music can also provide stimulation for premature infants to behave in ways that boost their development and keep them alive. Lullabies can serve as a low-risk source of stimulation and reinforcement for increasing nipple sucking (feeding) rates, providing infants with the nutrition they require for growth and development. Lullabies are thus associated with encouraging the rapid development of the neurological system and with a shorter length of hospitalization.

More recent research has shown that lullabies sung live can have beneficial effects on physiological functioning and development in premature infants. The live element of a slow, repetitive entrained rhythm can regulate sucking behavior. Infants have a natural tendency to entrain to the sounds that surround them. Beat perception begins during fetal development in the womb and infants are born with an innate musical preference. The element of live breathing sounds can regulate infant heart rate, quiet-alert states, and sleep. Live lullabies can also enhance parent-child bonding, thus decreasing parental stress associated with the intensive care. In short, live lullabies sung by music therapists induce relaxation, rest, comfort, and optimal growth and development.

Many lullabies, regardless of the meaning of their words, possess a peaceful hypnotic quality. Others are mournful or dark, like a lament. The Gaelic lullaby "Ba, Ba, Mo Leanabh Beag" was written in 1848 during the potato famine, which caused much hardship in the Scottish Highlands. The song mentions, soft potatoes, the mother's situation, and her fears for her child. In the 1920s, poet Federico García Lorca studied Spanish lullabies and noted the "poetic character" and "depth of sadness" of many of them. Lorca's theory was that a large part of the function of the lullaby is to help a mother vocalize her worries and concerns. In short, they also serve as therapy for the mother.

Combined with lament, lullaby can have "restorative resounding" properties for hospice inpatients and their families. Lullabies typically soothe people through the awake/sleep transition, and similarly can soothe people through the life/death transition. Music therapists have called these tunes "lullaments", that which sustain the spirit, support psychological structure, and enable resilience during times of vulnerability to the effects of adversity. Lullaments are music-contextualized expressions of attachment and detachment, sadness/tears and happiness/laughter, privilege and loss, nurturance and grief, deterioration, stasis and moving forward.

Many Christmas carols are designed as lullabies for the infant Jesus, the most famous of them being "Silent Night". "Hush Little Baby" has been observed cross-culturally and is known to have a natural capacity for soothing and energizing infants, as well as nurturing caregiving bonds. 

Mother–infant interaction
Infants exhibit a natural preference for infant-directed over non-infant-directed lullabies and their own mothers' voice over that of another female.

Ellen Dissanayake's emotional conjoinment hypothesis discusses the potential role of evolution in music ability. Mothers possess the enjoyment and capacity of producing musical notes and singing infant-directed songs, such as lullabies, to their infants. Dissanayake hypothesizes that early affiliative interactions between mother and child, which foster and coordinate emotions through rhythmic and temporally patterned vocal, body, and facial movements, lead to an emotional communion among participants, that gives rise to human music. The affiliative interactions, or relational bonds, between mother and infant, originally developed in primates during hominization, in response to increasing altriciality of their young. The modern function of human music, and the temporal arts as a whole, reflects these evolutionary origins – to bond and conjoin humans together on a physical and psychological level, in a species that requires close cooperation to win the competition for survival. Early on, mother and infant "practice and perfect their attunement" by mutually engaging in vocal, body, and facial movement that they construct together – movement with a particular duration, emotional expression, and rhythm (such as vocal phrases and pauses, sounds and silences). Conjoinment leads to sustained positive affectual states of interest and joy, fostered in both the infant and mother, important for sharing, communication, reinforcement, and motivation.

Much research has been generated on the role of lullabies in nurturing caregiving bonds between mother and child. Mothers who sing lullabies to their infants engage in a bonding activity that actually alters the underlying neural structure of the infant brain such that the infant becomes "tuned" into music and its association with parental affiliation. In one Taiwanese study of Kangaroo Care, a technique practiced on newborn infants in which a mother holds her child tightly against her chest, it was demonstrated that infant–mother dyads who listened to their choice of lullaby were associated with more quiet sleep states and less occurrence of crying by the infant and were also associated with significantly lower maternal anxiety, than those dyads who did not listen to lullabies. The therapeutic effect of lullabies can thus have a strong impact on calming anxieties and nurturing bonds, which is especially important with premature and fragile infants. 

In classical music
Lullabies written by established classical composers are often given the form-name berceuse, which is French for lullaby, or cradle song. The most famous lullaby is the one by Johannes Brahms ("Wiegenlied", 1868). While there has been no confirmation, there are many strong arguments that Brahms suffered from a sleep disorder known as sleep apnea. It is speculated (based on lullabies' utility as a sleep aid) that this was part of his inspiration for composing "Wiegenlied."

Chopin's Opus 57 is a berceuse for solo piano. Other famous examples of the genre include Maurice Ravel's Berceuse sur le nom de Gabriel Fauré for violin and piano; the Berceuse élégiaque by Ferruccio Busoni; the Berceuse from the opera Jocelyn by Benjamin Godard; the Berceuse by Igor Stravinsky which is featured in the Firebird ballet, and Lullaby for String Quartet by George Gershwin. The English composer Nicholas Maw's orchestral nocturne, The World in the Evening, is subtitled "lullaby for large orchestra". German composer's Paul Graener last movement of his Suite From The Realm of Pan is entitled Pan sings the world a lullaby. American composer's Michael Glenn Williams Berceuse for solo piano uses an ostinato similar to Chopin's but in a 21st-century harmonic context.

In popular music, famous lullabies include "Good Night" by The Beatles, "Lullaby" by The Cure, "Lullabye (Goodnight, My Angel)" by Billy Joel, "MLK" by U2 and "No Surprises" by Radiohead.