IT IS a time parents relish: their child's afternoon nap. But it seems that napping may not be such a good idea after all. Preliminary studies suggest that daytime napping in young children may be linked to poorer sleep and mental functioning than in their peers who only sleep at night. The big question is whether napping is the cause of the problem, or the result.

John Harsh at the University of Southern Mississippi in Hattiesburg and his colleagues asked the parents of 738 children aged between 2 and 12 about their children's sleeping habits. Children who took long daytime naps fell asleep at night an average of 39 minutes later and slept later at the weekend than those who did not nap. The effect was more pronounced in older children (over a quarter of 10 to 12-year-olds still took afternoon naps).The problem came during the following week, when children had to wake up at set times to get to school or to meet the demands of their parents' work schedules. The napping children continued to stay up later, meaning they spent less time in bed at night than their counterparts. "Napping children not only had a difficult time getting to bed, they had a harder time falling asleep, and they had a harder time getting up in the morning," says study author Alyssa Cairns, who presented the work at the annual meeting of the Associated Professional Sleep Societies in Minneapolis earlier this month.

The findings are backed by a study by Kazuhiko Fukuda of Fukushima University in Japan. He compared children who attended all-day pre-schools in Japan, where 90-minute naps are compulsory, with children of the same age who napped only when they needed to. As well as going to bed an average of 30 minutes later, the children who took obligatory naps were more likely to be moody in the morning and resist going to school, according to their parents. These behaviours lasted even after the children moved on to elementary school and stopped napping, perhaps because of the lasting influence of napping on their sleep and wake cycles, Fukuda suggests (Sleep and Biological Rhythms, vol 2, p 129).

Napping may also affect mental performance, according to Joe McNamara and his colleagues at the University of Florida in Gainesville, who also presented their results in Minneapolis. McNamara measured how well 27 kindergarteners could solve puzzles that measure planning and organisational skills. Children who took longer naps completed fewer puzzles successfully, and the later they went to bed, the less well they performed.

These findings pose a chicken and egg problem, says Harsh. "It could be that children are getting less sleep at night because they're napping, or they could be napping because they're getting less sleep at night," he says. Nevertheless, though napping and non-napping children in McNamara and Harsh's studies slept for the same amount of time in total "napping is not a substitute for night-time sleep", McNamara says.

父母們往往喜歡讓孩子們午睡，可現在看來，午睡恐怕并不像他們以為的那么有益。以往的研究發現，白天打盹的兒童比起沒這習慣的孩子，往往睡得更差，大腦機能也更弱。問題是，在這種關聯中，午休習慣究竟是因，還是果。

南密西西比大學John Harsh博士和他的同事們詢問了738名2至12歲兒童的父母，了解他們孩子的睡眠習慣。有長時間午睡習慣的小孩比不午睡的孩子晚上入睡平均晚39分鐘，周末則睡得更遲。這在較大的孩子中體現得更明顯（有四分之一10至12歲兒童仍然午睡）。當周末過去，因為自己上學和父母工作，孩子們沒有懶覺可睡，這時問題就出現了：有午睡習慣的孩子晚上依然睡得更遲，結果他們的睡眠時間也就更短。“午睡的孩子不僅很難哄上床，入睡和起床也都更不容易。”Alyssa Cairns說。Alyssa Cairns本月早些時候在明尼阿波利斯的專業睡眠協會年會上發表過研究成果。

日本福島大學福田一彥博士的一項研究支持了這些發現。他比較了在日本有90分鐘午休要求的全日制幼兒園的兒童和其他只在需要時打盹的同齡兒童。根據父母們的反饋，每天午睡的兒童晚上同樣平均晚睡30分鐘，早上起床上學時也更容易鬧脾氣，表現得很抗拒。甚至當這些孩子升入小學，不再每天午睡，仍然如此。福田認為，這也許是午睡習慣對他們生物鐘的持續影響使然。

午睡還可能影響智力表現。同樣在明尼阿波利斯發表過成果，來自福羅里達大學的Joe McNamara及其同事對此有所研究。McNamara為27名學齡前兒童安排了針對計劃與組織能力的難題，評價他們的表現。結果午睡時間較長的孩子普遍解出較少，而且晚上睡覺越晚的孩子往往表現得越差。

Harsh說，這些發現提出的還是一個先有雞還是先有蛋的問題。他說：“可以說孩子們晚上睡得少是因為白天打過盹，也可以說他們之所以白天打盹是因為晚上沒睡夠。”McNamara和Harsh統計過的孩子，不論有無午睡習慣，一天的睡眠時間還是大致相當的，按McNamara的話說，“在白天打個盹是不能替代夜間的睡眠的”。