Next day sleepiness and ability to concentrate is important!
January 20, 2016

Next day sleepiness and ability to concentrate is important!

In two independent field-intervention experiments, bedroom air quality was improved and the effects of the interventions on sleep, next-day questionnaire responses, and next-day performance were assessed. Opening a window in the pilot experiment will have allowed noise from outside the building to disturb sleep rather more than in the closed-window condition and will have allowed any wind to increase draughts, but these changes would be expected to reduce the beneficial effect of improved air quality on sleep, so the beneficial effects observed in the open window condition can be attributed to the change in bedroom air quality. In the main experiment, bedroom air quality was improved covertly, with no perceptible change in bedroom noise or draught, so any differences between conditions may confidently be attributed to improved bedroom air quality. There is no doubt that both interventions did improve bedroom air quality – the effective outdoor air supply rate was found to be greater by a factor of at least 10 if the window was open and by a factor of at least four if an air supply fan was covertly operated whenever the CO2 concentration was above 900 ppm. This led to a significant improvement in the subjects’ own ratings of perceived air quality (PAQ).

Improving bedroom air quality was hypothesized to improve sleep, and objective measures of sleep obtained by analyzing actigraph data confirmed this hypothesis, extending the findings of Lan et al. (2013 op.cit.) and Zhou et al. (2014 op.cit.) from personal ventilation to bedroom ventilation. It was possible to show that responses to the well-established Groningen Sleep Quality scale indicated that sleep quality improved with bedroom air quality and that the subjects’ own rating of next-day sleepiness and ability to concentrate differed significantly between conditions in the expected direction. Given these findings, it is reasonable to hypothesize that next-day performance would be better after sleeping in the conditions that provided better bedroom air quality. It was possible to show that this was the case, apparently for the first time. The size of the IAQ effect was about 3%. However, considerably more research is required before this preliminary finding in a quiet area with clean air can be generalized from students to the general population and to other climatic regions. There were no significant differences between conditions in perceived T or RH, yet in both experiments, subjects reported significantly more symptoms that are normally attributed to dry air in the better ventilated condition. In the open-window experiment, there was an almost significant tendency for subjects to report more air movement (two-tail P < 0.0555), but this was not found in the experiment in which a small fan was operated intermittently, nor would it be expected, so air movement is not an adequate explanation. Objectively measured RH did not fall below 40% in either condition, so it seems unlikely that the decrease from 52% or 54% caused by the intervention could have been responsible for the associated increase in reported skin, lip, and mouth dryness.

As increasing the outdoor air supply rate undoubtedly increased the concentration of whatever pollutants were present in outdoor air, including ozone, this may have been the cause of these symptoms. It should be remembered that bedroom air temperatures varied widely between subjects but did not differ between conditions and that symptoms attributed to air dryness can often be alleviated by reducing the air temperature.

Read an interesting article by P. Strøm-Tejsen, D. Zukowska, P. Wargocki, and D. P. Wyon published in the Journal of INDOOR AIR, doi:10.1111/ina.12254. The article reports two field intervention experiments on how the effective ventilation rate in bedrooms affects sleep and next day performance.

The effects of bedroom air quality on sleep and next-day performance (link here).