A Canadian survey found that 39% of respondents preferring the “log” position (lying on one’s side with the arms down the side) and 28% preferring to sleep on their side with their legs bent.

A Travelogue survey found that 50% of heterosexual British couples prefer sleeping back-to-back, either not touching (27%) or touching (23%). Spooning was next, with the man on the outside 20% of the time vs. 8% with the woman on the outside. 10% favored the “lovers’ knot” (facing each other with legs intertwined), though all but 2% separated before going to sleep. The “Hollywood pose” of the woman with her head and arm on the man’s chest was chosen by 4%

Health issues

In the 1958 edition of his best-selling book The Common Sense Book of Baby and Child Care, pediatrician Dr Benjamin Spock warned against placing a baby on its back, writing, “if [an infant] vomits, he’s more likely to choke on the vomits.” However, later studies have shown that placing a young baby in a prone position increases the risk of sudden infant death syndrome (SIDS).

A 2005 study concluded that “systematic review of preventable risk factors for SIDS from 1970 would have led to earlier recognition of the risks of sleeping on the front and might have prevented over 10,000 infant deaths in the UK and at least 50 000 in Europe, the USA, and Australasia.”
Pregnancy.org champions sleeping on one’s side, particularly the left side, for pregnant women, claiming this “will increase the amount of blood and nutrients that reach the placenta and your baby.”

A couple of research papers also link sleeping on one’s back during pregnancy with lower birth weight and increased risk of stillbirth, but these results must be corroborated by other larger studies.
It is recommended that people at risk of obstructive sleep apnea sleep on their side and with a 30° or higher elevation of the upper body. Snoring, which may be (but is not necessarily) an indicator of obstructive sleep apnea, may also be alleviated by sleeping on one’s side.

Directions

The Chinese Feng Shui and Indian Vastu Shastra systems describe favorable and unfavorable geographical directions (north, south, east, west) for sleeping. Feng shui also factors in the configuration of the bedroom in the positioning of the bed.

Modern scientific studies have suggested a beneficial effect of right lateral decubitus position on the heart. In particular, one study assessed the autonomic effect of three sleep positions (supine, left lateral decubitus, and right lateral decubitus) in healthy subjects using spectral heart rate variability analysis.The results indicated that cardiac vagal activity was greatest when subjects were in the right lateral decubitus position.