EFFECTS OF COLOUR OF LIGHT ON NONVISUAL PSYCHOLOGICAL PROCESSES IGOR KNEZ University of GÌvle, Sweden Abstract This study investigated the in£uence of colour of light (‘warm’, ‘cool’ and arti¢cial ‘daylight’ white lighting) on subjects’ self-reported mood, cognitive performance and room light estimation. Although no direct e¡ect on positive and negative mood was indicated, a main e¡ect of gender on several room light estimation dimen- sions showed that females perceived the room light, across all light settings, as more expressive than did males. Cognitive task data revealed ¢ndings not earlier indicated by the behavioural lighting research. Main e¡ects of colour of light on short-term memory and problem solving showed that subjects performed better in the ‘warm’ than in the ‘cool’ and arti¢cial ‘daylight’ white lighting. Interaction e¡ects between colour of light and gender on long-term memory showed that males performed best in the ‘warm’and ‘cool’ white lighting, and that women performed better than men in the arti¢cial ‘daylight’ white lighting. All these results are generally in agreement with the proposition that light in£uences nonvisual psychological processes. # 2001 Academic Press Introduction Knowledge of light e¡ects on humans has tradition- ally been based on the visual perception research, on the whole, addressing issues of visual task per- formance and discomfort in di¡erent light settings (e.g. Boyce, 1981; Vos, 1984). Recent research has, however, attempted to establish the nonvisual in£u- ence of light on basic biological and psychological mechanisms. Most convergent evidence, so far, for the nonvisual e¡ects of light on humans emerges from (chrono)biological research and indicates a light sensitive endogenous circadian clock regulat- ing melatonin secretion (e.g. Lewy et al., 1980; Arendt & Broadway, 1987; Brainard & Bernecker, 1995) and possible light transduction through the skin (Campbell & Murphy, 1998). Based on a devel- oping understanding of relation among seasonal light change and circadian desynchronization, light therapy is used to treat a variety of depressive dis- orders (e.g. Dalgleish et al., 1996; Lam et al., 1989). If we leave preference studies out of account, empirical psychological research has generated di- vergent evidence for the nonvisual e¡ect of light. Several studies have indicated e¡ects of light on attention (Giusa & Perney, 1974), perceived guilt (Aspinall & Dewar, 1980), time-estimation task per- formance (Delay & Richardson, 1981), interpersonal communication (Gi¡ord, 1988), mood (Belcher & Kluzny, 1987b), performance of various cognitive tasks and interpersonal behaviours (Baron et al., 1992), mood and decision making (McCloughan et al., 1996) and self-reported quality of life (S˛rensen & Brunnstr˛m, 1995). Still others have failed to ¢nd direct e¡ects of light on mood (Baron et al., 1992) and on performance of various cognitive tasks and mood (Boray et al., 1989; Veitch et al., 1991; Veitch, 1997). Knez (1995) sketched an experimental framework suggesting mechanisms operating for psychological nonvisual light e¡ects. In the light of an extensive quantity of evidence showing e¡ects of physical fac- tors on emotion (e.g. Evans, 1982; Russell & Snod- grass, 1987) and of evidence showing an in£uence of emotion on cognitive processes (e.g. Clark & Fiske, 1982; Isen, 1984), Knez proposed, akin to Belcher and Kluzny (1987a) and Baron and Rea (1991) thinking, a causal link of a¡ect from the lumi- nous environment to cognitive performance via mood. Knez and associates (Knez, 1995; Knez & Enmarker, 1998; Knez & Kers, 2000) reported a series of factorially designed (e.g. Kirk, 1968) Journal of Environmental Psychology (2001) 21, 201^208 0272 - 4944/01/020201 + 08$35.00/0 # 2001 Academic Press doi:10.1006/jevp.2000.0198, available online at http://www.idealibrary.com on