The conclusion in their report seen by The Scotsman comes as members of the Rail, Maritime and Transport union (RMT) prepare to stage a second 48-hour walkout which will force the cancellation of some services on Sunday and Monday nights.
Staff voted to take action after operator Serco refused to allow them to use spare cabins for rest breaks during shifts which can last more than 16 hours.
The report by fatigue specialist Dr Paul Jackson concluded the firm complied with health and safety laws but its “approach to fatigue management could be improved”.
It said staff breaks should be extended beyond one hour on longer routes, rest areas improved and the working of multiple nights on end be reduced.
The report stated: "Of the available options, a berth provides the best conditions for rest away from interruptions, as well as affording the best opportunity to obtain proper rest.”
Dr Jackson highlighted faults with the new Sleeper fleet as a major cause of extra staff stress, especially when they had to find alternative cabins for arriving passengers at short notice.
Problems on a London-Fort William service observed by his team in August included with air conditioning, toilets and doors.
The report stated: "It was clear that the extra workload arising from factors outside crews’ control causes unnecessary stress. A poor start to a long night duty.”
It concluded: “Fatigue management initiatives have been developed, but have not always been adequately documented, or communicated to the workforce, leaving many crew with the impression that management don’t understand their situation.
"Positive initiatives are also undermined by one of the main causes of crew stress and workload – the ongoing issues with train defects.
“A change in culture is required at Serco Caledonian Sleeper, focusing on alertness management rather than fatigue.”
The RMT described the report as “damning” and said it supported staff concerns.
The union said: “It shows fatigue has been an issue for years but has been allowed to fester, staff are reluctant to report fatigue for fear of victimisation, and train faults are rife, which adds to the stress.
“We want the report to be implemented in its entirety plus the provision of two more berths for rest breaks – that would settle the dispute.”
Caledonian Sleeper managing director Ryan Flaherty said: “When some of our employees raised concerns about fatigue, we brought in an independent specialist to conduct a risk assessment on our services.
"His report clearly concludes that not only is Caledonian Sleeper operating within all regulations, but that only two people raised the issue of additional rooms for hosts, which has become the RMT’s main demand."
The firm said it planned further discussions with the RMT “to agree specific actions relating to the recommendations” to resolve the dispute but the union had made this conditional on being given the extra cabins.
Serco has cancelled its London-Glasgow-Fort William/Inverness services on Sunday and Monday because of the strike but will operate its London-Edinburgh services.