MICROSLEEP A small sleep... a BIG problem A paper presented to the International Railway Safety Conference Tokyo 2002 by Dennis Bevin Rail Accident Investigator
1. INTRODUCTION The incidents being used as case studies resulted in the following: • WestmereTwo locomotives at the head end of a train rolled and five trailing wagons were derailed. Kai Iwi Ten loaded milk tankers rolled and slid down an embankment. MiddletonTwo locomotives and several wagons were damaged and derailed as a result of a head on collision.
2. BACKGROUND • “A brief unintentional period of light sleep, lasting from seconds to minutes, during which the eyes may be open but the brain is disengaged and is not processing information. The causes of microsleeps are directly related to fatigue”. What is Microsleep?
2. BACKGROUND Method for assessing fatigue Fatigue assessment was based on a method developed by the US National Transportation Safety Board and the NASA Countermeasures Program. This method seeks information on the following factors known to produce fatigue-related performance impairment: • Extended wakefulness. • Acute sleep loss and cumulative sleep debt. • Presence of a sleep disorder. • Critical times in the daily cycle of the circadian body clock.
3. CASE STUDIES Westmere and Kai Iwi • An express freight train derailed near Kai Iwi while rounding a curve about 25 km/h faster than the authorised and posted curve speed of 50 km/h. An express freight train derailed when travelling too fast for the first curve encountered descending a 1 in 35 grade near Westmere. The reason for the excessive speed in both cases was the LE’s loss of awareness during a microsleep.
3. CASE STUDIES Westmere and Kai Iwi Safety issues identified included: • The control of LEs hours. • Fatigue management. • The ability of the locomotive’s vigilance system to overcome a short-term attention deficit in sufficient time to allow effective corrective action to be taken.
3. CASE STUDIES Middleton • The passing of the signal resulted from the LE’s loss of attention and situational awareness consistent with his having fallen asleep. A northbound express freight train passed Signal 212 at Middleton at “Danger” and collided head-on with a departing southbound express freight train. Safety issues identified again included: • The control of LEs hours. • Fatigue management. • The ability of the locomotive’s vigilance system to overcome a short-term attention deficit in sufficient time to prevent this type of collision.
Signal 212 Middleton yard Signal 202 up main line from Rolleston to Addington from Addington down main line to Rolleston route set for Train 951 3. CASESTUDIES • Middleton • Setting the scene
POINT OF IMPACT 20 seconds from impact – while travelling at 58 km/h the LE has become fully aware of the situation and applied the emergency brakes. 112 seconds from impact, travelling at 56 km/h, the LE cancelled the visual vigilance device by moving the throttle from notch 6 to notch 5. 90 seconds from impact, travelling at 57 km/h Train 828 passed Signal 1712 at yellow. 56 seconds from impact and travelling at 59 km/h the LE cancelled visual vigilance by operating throttle (automatic reflex?). 54 seconds from impact and while travelling at 59 km/h the LE entered a microsleep 43 seconds from impact and travelling at 59 km/h the LE had a memory of Train 828 emerging from under overbridge. • Middleton • Time sequence of events up main line Signal 212 down main Overbridge
3. CASE STUDIES Middleton • Relief shifts and standbys were built in to allow for annual leave, sickness and operational demands. The Locomotive Engineer’s roster LEs base rosters were compiled using defined principles of fatigue management, with fortnightly rostered shifts at or about 80 hours. Mini rosters were compiled for each LE and included changes to the base roster to accommodate staff availability and operational requirements. Actual hours worked could vary from those rostered due to late running and other operational factors on the day.
3. CASE STUDIES Middleton The Locomotive Engineer’s roster The mini-rostered hours and those actually worked by the LE prior to the incident are shown in the following table:
4. Factors that increase the likelihood of falling asleep uncontrollably • German study suggests that an LE’s vigilance is at its worst in the early hours of the morning. Time of day Biological sleepiness waxes and wanes across the daily cycle of the circadian body clock. People are most prone to falling asleep inadvertently in the early hours of the morning and again in mid - afternoon.
4. Factors that increase the likelihood of falling asleep uncontrollably • The German study also found that how long an LE had been on shift affected how impaired his alertness became in the early hours of the morning. Time on shift
4. Factors that increase the likelihood of falling asleep uncontrollably • The accuracy of information on an LE’s sleep history can be limited by the following factors: Duration of continuous wakefulness Laboratory studies consistently show that biological sleepiness increases the longer a person stays awake (sleep history). • Subjective reports of sleep duration and timing are not necessarily reliable. • The length of time from the first of the sleep episodes being recalled to the time of the interview. • The length of time between the incident and the interview by the investigator.
4. Factors that increase the likelihood of falling asleep uncontrollably • Getting 2 hours less sleep than they need on one night is enough to impair most people’s performance and alertness the next day. Prior sleep loss Insufficient prior sleep increases biological sleepiness at all times in the circadian body clock cycle. It typically takes 2 full nights for sleep and daytime functioning to return to normal after sleep loss.
4. Factors that increase the likelihood of falling asleep uncontrollably • Sleep that is restless and fragmented by frequent awakenings also leaves a person sleepy and at increased risk of impaired alertness and performance. Presence of a sleep disorder/quality of sleep The restorative value of sleep, in terms of reducing biological sleepiness and improving subsequent waking function, depends not only on the amount of sleep obtained but also on its quality.
5. ROSTERING ISSUES Forward rotation and short breaks between shifts • The amount of sleep that a person can obtain during a break is highly dependent on the time of day at which the break occurs. Forward rotations of shifts reduced the likelihood of very short breaks between shifts, which can restrict the time available for sleep.
5. ROSTERING ISSUES Late running • Westmere The LE’s previous 4 shifts had resulted in each train running an average of 1 hour 36 minutes late. Late running, particularly after night shifts, reduces the time available for sleep and can contribute to the accumulation of sleep debt across consecutive shifts. Kai Iwi The LE’s previous 4 shifts had resulted in trains running an average of 55 minutes late. Middleton Of the LE’s previous 5 shifts 4 had resulted in trains running an average of 38 minutes late.
5. ROSTERING ISSUES • There can be numerous reasons why LEs agree to work extra shifts over and above those for which they are originally rostered. Working on rostered days off
5. ROSTERING ISSUES • concern about possible effects of refusal on relationships with other locomotive engineers, or with the company These include: • remuneration incentives • loyalty to fellow locomotive engineers at the depot, who may be less well-rested or have important commitments away from work • professional motivation to ensure that the system runs smoothly • loyalty to the company.
5. ROSTERING ISSUES Working on rostered days off • Westmere The LE was called back on a rostered day off duty between two night shift cycles to work a day shift. Additional shifts prior to a block of night shifts prevent an LE from being well rested going into the night shifts. Middleton The LE had worked both of his rostered days off the preceding weekend and both of these extra shifts had involved night work.
5. ROSTERING ISSUES Opportunities for recovery from sleep debt • Westmere The LE had been called back on a rostered day off between two night shift cycles to work a day shift which, although it did not directly restrict his nighttime sleep, would have restricted his opportunities for rest and recovery between the night shift cycles. For daytime functioning to return to normal after sleep loss it typically takes 2 full nights of sleep. Middleton In the month preceding the collision the LE had been rostered off duty for one 4-day block during that time but had worked day 2 and day 4 of that block. He had also been rostered off duty for one further 2-day block but had worked both of these days. In that time he had only 1 break of at least 48 hours free from work.
6. FINDINGS The following factors were identified from the 3 investigations. The number in brackets indicate the number of investigations to which each finding was relevant: • The LE lost attention and situational awareness as a result of falling asleep through fatigue (3) • The LE was probably experiencing the effect of an accumulated sleep debt at the commencement of his shift (3) • The accident occurred at a time when the LE’s biological sleepiness would be expected to be increasing rapidly towards its daily maximum (3)
6. FINDINGS • The LE’s increase in sleepiness due to the daily cycle of his body clock would have been exacerbated by his prior sleep loss, and by his being between three and a half hours and six hours into his shift (3) • The base rostered hours for the LE were excessive (2) • The mini rostered hours for the LE were excessive (2) • The actual fortnightly hours which would have been worked by the LE had the accident not happened would have been excessive (2)
6. FINDINGS • The LE responded to a request to work on his rostered days off duty in the days prior to his accident (2) • The current locomotive vigilance system did not provide an effective defence against short microsleeps (3) • The operator had no process in place to monitor and control total mini-rostered hours and actual hours worked each fortnight (3)
7. SUMMARY • This table compares aspects of the 3 case studies: * Denotes days prior to accident * denotes days prior to accident
7. SUMMARY • MiddletonThe LE fell asleep after passing a “caution proceed” signal, waking as he approached the next signal at “Danger”, but too late to stop his train before it collided with an oncoming train. Westmere and Kai Iwi The LEs fell asleep near the top of an ascending grade, and then did not brake.
7. SUMMARY • they occurred between 3 hours 30 minutes and 6 hours into a night shift that was between the 5th and 10th in a sequence of night shifts These incidents all had in common: • the LE’s preceding night shift had also run late • they all occurred at or near the daily peak in biological sleepiness
7. SUMMARY • the reduction of staff numbers or a general staff shortage which may require remaining staff to perhaps work longer and more often There are commercial factors which have the potential to increase the risk of microsleeps amongst LEs: • the introduction of single person crewing • the requirement for increased overnight services to match competitors’ service
8. CONCLUSION There has been traditionally a low level of reporting microsleeps on the part of LEs, due to: • they were unaware they had happened • fear of the Company’s response • underating the significance
8. CONCLUSION • Although the Commission has not investigated any previous accidents or incidents where microsleep was an obvious causal factor, there has long been anecdotal evidence from other investigations carried out that microsleeps were occurring frequently.
8. CONCLUSION For it’s part the operator in New Zealand is taking positive steps towards both educating staff and developing procedures to counter the causes of microsleeps: • It is developing an improved fatigue management program which it plans to put all Locomotive Engineers through • It has introduced new processes within the roster centre to monitor mini rostered and actual work hours on an ongoing basis • A joint working party between the Union and the Company has designed new rostering procedures which limit the number of consecutive night shifts that can be worked and defined the minimum rest period which must then be enforced.
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