From Sihui Yu:
A lullaby or cradle-song, also spelled cradle song, is a soothing song or piece of music, usually played or sung to children or adults. 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 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 qualitiesLullabies 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."