Generally speaking, jet lag, changes in environment, stress and so forth can cause temporary insomnia. If the condition doesn't last for more than a month, then it isn't too serious. It is more worrisome, however, when sleeping problems persist for over a month with no sign of improvement. This is "chronic insomnia," and it may strike three or four times a week.
一般而言，因時差、壓力、環境變化等造成的短暫或短期性失眠，時間短於一個月，問題不大，比較惱人的是失眠超過一個月未改善，一星期當中有三、四天睡不好的「慢性失眠」。