If by "In the past" you mean on a traditional tram then falling asleep would be more or less impossible as the drivers were mostly stood up and sometimes in the open.
The thing that strikes me is that we are hearing a lot about Croydon tram drivers but not about Metrolink, Nottingham, Sheffield and so forth. So surely it is worth considering whether the routes, the trams the working hours or some combination are especially conducive to this in Croydon.
The results of such a study should be useful to staff unions and management.
We're hearing a lot more about Croydon because people are paying more attention to drivers after last years fatal accident.