Based on the PROMIS (short form), sleep disturbance in the previous 4 weeks was assessed through four single questions scored on a four-point scale (0 for favourable answers, 1 for unfavourable answers) [14, 32]. An example of a given question ‘In the past 4 weeks, I had difficulty falling asleep’. Possible answers: not at all (0 points), a little bit (0 points), somewhat (0 points), quite a bit (1 point), very much (1 point). A total score ranging from 0 to 4 was obtained by summing up the answers to the four questions, a score of 2 or more indicating the presence of sleep disturbance. The PROMIS has been validated in several populations and languages including English, French and Spanish (internal consistency: >0.9; test–retest coefficients: 0.8; construct validity: product–moment correlations ≥0.9) (for detailed information, see www.nihpromis.org) [14, 32].