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.

父母们往往喜欢让孩子们午睡，可现在看来，午睡恐怕并不像他们以为的那么有益。以往的研究发现，白天打盹的儿童比起没这习惯的孩子，往往睡得更差，大脑机能也更弱。问题是，在这种关联中，午休习惯究竟是因，还是果。

南密西西比大学John Harsh博士和他的同事们询问了738名2至12岁儿童的父母，了解他们孩子的睡眠习惯。有长时间午睡习惯的小孩比不午睡的孩子晚上入睡平均晚39分钟，周末则睡得更迟。这在较大的孩子中体现得更明显（有四分之一10至12岁儿童仍然午睡）。当周末过去，因为自己上学和父母工作，孩子们没有懒觉可睡，这时问题就出现了：有午睡习惯的孩子晚上依然睡得更迟，结果他们的睡眠时间也就更短。“午睡的孩子不仅很难哄上床，入睡和起床也都更不容易。”Alyssa Cairns说。Alyssa Cairns本月早些时候在明尼阿波利斯的专业睡眠协会年会上发表过研究成果。

日本福岛大学福田一彦博士的一项研究支持了这些发现。他比较了在日本有90分钟午休要求的全日制幼儿园的儿童和其他只在需要时打盹的同龄儿童。根据父母们的反馈，每天午睡的儿童晚上同样平均晚睡30分钟，早上起床上学时也更容易闹脾气，表现得很抗拒。甚至当这些孩子升入小学，不再每天午睡，仍然如此。福田认为，这也许是午睡习惯对他们生物钟的持续影响使然。

午睡还可能影响智力表现。同样在明尼阿波利斯发表过成果，来自福罗里达大学的Joe McNamara及其同事对此有所研究。McNamara为27名学龄前儿童安排了针对计划与组织能力的难题，评价他们的表现。结果午睡时间较长的孩子普遍解出较少，而且晚上睡觉越晚的孩子往往表现得越差。

Harsh说，这些发现提出的还是一个先有鸡还是先有蛋的问题。他说：“可以说孩子们晚上睡得少是因为白天打过盹，也可以说他们之所以白天打盹是因为晚上没睡够。”McNamara和Harsh统计过的孩子，不论有无午睡习惯，一天的睡眠时间还是大致相当的，按McNamara的话说，“在白天打个盹是不能替代夜间的睡眠的”。