Lighting and self-regulation : can light revitalise the depleted ego

Recent research has demonstrated that light increases alertness and performance on cognitive tasks even during daytime (Phipps-Nelson et al. 2003; Smolders & de Kort, submitted; Vandewalle et al., 2006). Neuroimaging has indicated that light during daytime – in particular after sleep deprivation - increases activity in the thalamus and prefrontal cortex (Vandewalle et al., 2006). The prefrontal cortex is the area where executive functioning is said to reside, and perhaps also self-regulation (Wagner & Heatherton, 2011). After earlier experiments demonstrated improvements in performance on vigilance tasks during an hour long exposure to bright light (Smolders & de Kort, submitted), we now wanted to test the effect of brief lighting exposure on replenishment after ego-depletion. Ego depletion is the term used for the mental state after exerting self-control. Research has shown that performance on subsequent tasks is impacted after ego-depletion, indicating that self-regulation relies on a limited resource (e.g., Baumeister et al., 1998). Considering the effects of light on performance and brain activation we hypothesized that bright and/or blue light might help replenish this limited resource. We therefore designed a study to test such effects. In a pilot study (N=64), we depleted participants’ resource for self-regulation by asking them not to eat from a dish of freshly baked muffins. We then offered them a 1-2 minute light treatment of a high (6000K) vs. low (3000K) colour temperature. Immediately following this light treatment, they performed the grip test, of which a baseline measurement was taken upon entering the lab, and completed a short questionnaire assessing mood. As control measures we used self-reports of perceived lighting characteristics and evaluation, and beliefs concerning effects of light on physical and cognitive performance. Initial results showed non-significant trends of the colour temperature manipulation on changes in subjective vitality (p=.07) and duration of the grip test (p=.11). However, if we selected only the participants (N=41) who accepted the muffin we offered at the end of the experiment (a behavioural indicator for their appetite for muffins and hence an indirect indicator of how taxing the depletion induction was), the effect of the light manipulation was significant, both on subjective vitality (F=6.79, p=.01) and on duration of the grip test (F=4.35, p=.04). Persons in the high CCT group improved their performance on the grip test (M=12.3), whereas performance in the low CCT group worsened (M=-.15.8). For the actual experiment we will employ a different ego-depletion induction and the design will also include the non-depleted conditions for both light settings. The experiment is currently being planned; results will be presented at the conference.