CPD PROGRAMME | LIGHTING and heart disease. As the population ages, an increasing number of elderly people suffer with conditions related to a failing circadiansystem and their consequent sleepproblems. Light, therefore, not only affects what is consciously visible, but also alertness, wellbeing and performance. Although the human circadian rhythm and seasonal variation are genetically fixed, they will be affected by the human condition, and are likely to be influenced by the surroundings including the visual environment. As the sun rises in the morning and descends at night, there is a continuous progressive variation in colour range and intensity. Daylight, with a high proportion of blue light particularly in the morning contributes to the suppression of the hormone melatonin (which promotes sleep), reducing the prospect for deep sleep. The hormone cortisol is produced in the body in the morning, stimulating the metabolism, programming the body for the days activity. The first morning light helps suppress the production of melatonin, while the levels of cortisol decrease over the course of the day. For daytime workers, it can be desirable to support the activity of the cortisol at the beginning of the working day by stimulating cold-tone light and high illuminance values. In the afternoon, the production of cortisol in the body has already decreased significantly. The production of the sleep hormone is not yet required as the evening approaches, and practice has shown that, in this transitional period, alertness levels can be enhanced by means of a neutral mixed light and reducing the illuminance level. In the evening, as people tire, the melatonin levels in their blood increases. Then, as they sleep, melatonin levels drop in readiness for morning awakening. A lack of light during the day and too much in the evening has been shown to adversely affect sleeping patterns. As discussed in the SLLs Code for lighting, in recent years evidence has accumulated for the presence of a non-visual photoreceptor in the human eye, the primary purpose of which is to regulate the bodys internal clock. This has begun to reveal the importance of exposure to adequate daylight levels for the health and wellbeing of humans, many of whom now spend most of their lives inside buildings. Light sensitivity is controlled by rods and cones in the eye. Rods deal with low sensitivity and cones deal with colour and higher sensitivity. The recently recognised third receptor the intrinsically photosensitive retinal ganglion cell may provide an important means of linking health and wellbeing to light. 3,000K warm white 4,500K cool white 6,500K daylight white Figure 1: Illustration of colour temperatures (Source: Ridi Group) The benefits of different hues of white light have been well documented for example, the blue end of the spectrum is linked with alertness. However, it is still not clearly understood what other factors in the lighting scene are significant. Studies indicate that higher lighting levels at the beginning of the morning that slowly decrease, together with a gradual increase in colour temperature, may be beneficial. However, the physiological effects of tuneable white light need careful Human-centric lighting in practice At the Amsterdam office of the property company CBRE (Figure 2), a time-controlled lighting system features a circadian-friendly lighting sequence, which varies the colour temperature and intensity during the course of the day. Employees are stimulated during the morning and early afternoon with high illuminance levels and cool indirect white light. At midday and late afternoon, the light levels fall and become warmer. It was reported that researchers found an accuracy improvement of 12% in an objective experiment. Additionally, the participants working in the office with the human-centric lighting (and other novel design features) found their total work performance to be 18% better, 71% found they had more energy, 76% thought they were happier, and 50 per cent said they werehealthier. 20 December 2018 www.cibsejournal.com CIBSE Nov18 Supp pp19-22 CPD 139 v2.indd 20 23/11/2018 12:59