Identifying the causes of sleepiness in various safety-critical work environments is necessary for implementing more efficient fatigue management strategies. In transportation, little is known about drivers’ own perceptions of these causes. Therefore, we instructed shift-working tram (n = 23) and long-haul truck drivers (n = 52) to report at the end of their shifts what made them sleepy if they felt so. These self-reports, measured on-duty sleepiness, and sleep amounts were recorded on every shift over a period of 2–3 weeks per driver. The causes of sleepiness were queried with smartphone applications and sleep logs. Sleepiness was measured with the Karolinska Sleepiness Scale (KSS) and sleep with wrist-worn actigraphs. Data were analyzed using generalized estimating equations. Sleep loss and insufficient rest breaks were commonly reported as causing sleepiness among the tram drivers, whereas time of day and sleep loss were the leading causes among the truck drivers. Other causes, such as traffic or cabin conditions, were not frequently mentioned. During morning, day, and evening shifts, the truck drivers were less likely to report insufficient rest breaks as causing sleepiness than the tram drivers. Similarly, during morning shifts, the truck drivers were less likely to attribute their sleepiness to sleep loss. In shifts with drives reporting severe sleepiness (KSS ≥ 7 at least once, 18–21% of shifts), sleep loss was significantly reported as causing sleepiness among both groups. Reporting insufficient rest breaks was associated with severe sleepiness among the tram drivers, whereas time of day showed the same among the truck drivers. The results highlight the need for addressing sleep-related fatigue in transportation and provide directions for future research with regard to secondary causes of sleepiness.

为了实施更有效的疲劳管理策略，必须在各种对安全至关重要的工作环境中找出导致困倦的原因。在运输中，对驾驶员自己对这些原因的看法知之甚少。因此，我们指示轮班工作的有轨电车（n = 23）和长途卡车司机（n = 52）在他们轮班结束时报告他们是否感到困倦。这些自我报告，测量的值班嗜睡情况以及每个驾驶员在2-3周内每次班次的睡眠量均被记录下来。通过智能手机应用程序和睡眠日志查询了嗜睡的原因。使用Karolinska嗜睡量表（KSS）测量嗜睡，并使用腕戴式活动记录仪测量睡眠。使用广义估计方程分析数据。据报道，睡眠不足和休息时间不足会导致电车司机嗜睡，而白天和睡眠不足是卡车司机的主要原因。很少提及其他原因，例如交通或客舱状况。在早上，白天和晚上的轮班中，卡车司机报告的休息时间不足引起的困倦的可能性低于电车司机。同样，在早上上班时，卡车司机不太可能将困倦归因于睡眠不足。在有严重睡眠报告的轮班中（至少一次KSS≥7，占轮班的18–21％），两组均报告有严重的睡眠障碍。报告休息时间不足与电车司机严重困倦有关，而一天中的时间在卡车司机中也一样。结果强调了解决交通运输中与睡眠有关的疲劳的需求，并为以后关于嗜睡的次要原因的研究提供了方向。