The effect of a bed-partner on sleep was investigated by separating the sleep data, on the basis of the sleep logs, into three categories: (i) no partner; (ii) partner sleeping in a different bed and (iii) partner sleeping in the same bed. As the results may be confounded by age (because more of the older subjects slept alone) an ancova was performed with age as a covariate. As one may predict, sleep latency was longest for the subjects who were sharing their beds with a partner and shortest if the partner was in a different bed. Subjects who had a partner in another bed spent more time in stage 1 and less time in stage 4 when compared with the other two categories and, on this basis, could be described as poor sleepers, who may have chosen to sleep in separate beds.
In general, the results from this field study, in which subjects undertook their normal daily activities and maintained their habitual sleeping arrangements, are in good agreement with previous normative data for age and gender derived mainly from laboratory studies (Williams et al. 1974; Bliwise 1993). In addition, our data indicated no evidence of a FNE nor that our oldest group were more prone to disturbance due to a FNE or the serial night of recording. However, it is important to note that the mean age of the three groups reported here were 26, 40 and 60 y, which presents a greater focus on the middle years of life compared to previous studies. In most studies of sleep in the aged, the elderly subjects are usually well over 65 years. Our field data agrees with most previous laboratory studies showing that ageing from early adulthood through middle age into old age is associated with progressively less depth and continuity in sleep measures (Bliwise 1994).
Field studies are an important adjunct to laboratory based research, but can suffer because of the reduced control that can be employed, which may pose limitations on the generality of the findings. Two potential limitations present in this study were the local conditions, i.e. adjacent to airports, and the subject selection criteria.
Despite our subjects being drawn from sites near to Airports there was clear evidence (Horne et al. 1994) that only a small minority of aircraft noises (1.2%) affected sleep while there were more substantive domestic and idiosyncratic causes of disturbance such as small children and visiting the toilet. Similar, low levels of disturbance have been found for local residents in a field study (Fidell et al. 1995) which employed behavioural responses to confirm awakenings due to aircraft noise. Therefore, the EEG data gathered from these sites adjacent to airports can be considered representative of the wider population in the sense that subjects drawn from areas not exposed to aircraft noise, but similar in other respects, would exhibit similar sleep. However, it could be argued that the closeness of an airport would tend to ‘shape’ the residents by e.g. causing the more noise prone to move away and deterring potential house-buyers who may be light sleepers. This could result in a more sleep resilient type of individual residing near to airports.
However, not all the sites were very noisy, as the project design required a range of noise locations where the quietest (Stansted) had hardly any night flights and at others, night flying was restricted. In addition, the lack of sleep disturbance at the more noisy sites was most likely due, in part, to the measures taken by the residents to insulate their homes and bedrooms against sound e.g. double glazing, for which residents at the noisiest sites recieved financial aid from the airport. Also, two thirds of the subjects had lived in their houses for at least 5 years allowing potential habituation to be well established.
There were potential limitations in the selection criteria; subjects were not questioned about previous psychiatric illness e.g. depression or current sleep disorders e.g. sleep apnoea, which may differentially affect sleep measures as a function of age and/or gender. Also, subjects who suffered insomnia, from whatever cause including aircraft noise, may have resorted to the frequent use of hypnotics, and, similar to deaf subjects, would have been excluded from our subject group. However, only about 4% of potential subjects were excluded for these reasons.
Another potential effect of our selection process was the exclusion of individuals who suffered physical illness and pain which affected sleep. This could be criticised for introducing bias because the older subjects would be more likely to be excluded for this reason. This would have the effect of improving observed sleep in the older group. However, this restriction only affected about 2% of the potential sample.
The one major consideration that favours our sample being reasonably representative of a normal sleeping population was the consistent picture of the major effects of age and gender on sleep obtained in this field study with the bulk of normative sleep data in the literature from sleep laboratories. Therefore, despite our subjects living adjacent to airports, employing relatively limited selection criteria and the subjects maintaining their habitual lifestyle, their sleep was similar to that found in laboratory based control groups.