[1] Grampian Univ Hosp Trust, Aberdeen Royal Infirm, Dept Resp Med, Aberdeen AB25 2ZN, Scotland
[2] Oldchurch Hosp, Essex Ctr Neurol Sci, Dept Neurol, Romford RM7 0BE, Essex, England
[3] Royal Infirm, Scottish Natl Sleep Lab, Edinburgh, Midlothian, Scotland