When Human Centric Lighting gets personal: p-HCL

Professor Linnartz and Ph. D Charikleia Papatsimpa wrote a very interesting article in the latest LED professional Review. Light has an unquestionable effect on human health, physiology, comfort, productivity and mental wellbeing. Light influences our sleep/wake cycle, hormone secretion and subjective alertness and performance. We understand and can predict the effects of light more and more.

Photo by ludovic toinel on Unsplash


Light gives our biological clock an indication of time. The central biological clock maintains a temporal synchrony between internal periodic cycles and external environment that is thought to enhance overall organismal function and survival. But can we mimic this daylight with artificial lighting?


A wealth of insights is available from studies in controlled lab conditions. The non-image forming effects of light on circadian rhythms are even modelled as a set of different equations. This gave a starting point for their research. Future light recipes were tested which allowed the preselection of the optimal light recipe for the individual based on his/her particular needs, responses and sensitivity to light input. Different people have different needs and are exposing to different settings. A lighting recipe designed to make an evening person wake up earlier in the morning is not the same as the lighting recipe for a morning person.


It’s proven that light late in the evening and at night delays the circadian wake propensity rhythm, as a result people that are exposed to bright evening light have late spontaneous sleep and wake-up times. For an evening person that effect can be detrimental because they already have a clock that runs at a slower pace and receiving light at the wrong time of the day can shift their sleep schedule even later. This negative effect can to some extent be counteracted by increasing light during the day, especially during the morning. A morning person however should not be receiving light in the early morning, their clock is already in a phase where light delays the clock. They would need a late afternoon light boost.


During this research some important lessons were learned: one of those is that the impact of light exposure is a slow and subtle process, the effects of light on the specific users are preferably tracked over longer periods of time. Preferably, one should not aim to drastically shift their circadian phase and sleep cycles.

Read the full article here on p. 38

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