Lighting is known to be an important factor for controlling indoor environment. Light facilitates perception, can create a pleasant atmosphere and provides a powerful stimulus to our biological clock, thus supporting a healthy activity—sleep cycle.
The human circadian (24 hr) rhythm is accompanied by a 24-hour, almost sine wave-shaped, variation of the core body temperature (CBT) of the human body. The peak-to-peak value of the CBT variation is typically some 0.7 degrees centigrade. The CBT minimum usually occurs at night, around 1-2 hours before spontaneous wake-up. Nocturnal darkness is associated with a peak in secretion of the hormone melatonin. Melatonin reinforces darkness-related behavior, which for humans implies sleep. Sleep is associated with lower temperatures while activity is associated with higher temperatures. A temperature difference between distal skin (hands, feet) and proximal skin (thigh, stomach) may promote onset of sleep. For rapid sleep onset it is essential that the body can discharge heat by using distal skin regions to dissipate heat from the core body to the environment, allowing the core body temperature to drop. This demonstrates that thermoregulation can be used as a means to control sleepiness of an individual. Exposure to nocturnal light suppresses nocturnal melatonin secretion, thus influencing thermoregulation as the melatonin peak is usually associated with the minimum in CBT. By influencing the melatonin levels and phase shifting the biorhythm, light has an indirect influence on thermoregulation.
It is less well known that light also has a direct influence on thermoregulation in humans. Bright light exposure decreases the core body temperature, even during exercise. The higher the color temperature of the light source, the stronger this effect, although at high levels saturation of this effect may occur. The CBT lowering effect of bright light exposure may persist several hours after exposure has ended. Bright light exposure over several hours during the daytime appears to lower the CBT threshold above which cutaneous vasodilatation and forearm sweating occurs.
After daytime bright light exposure, subjects felt less cold during chilly afternoons or evenings. These findings indicate a reduced set point of core body temperature caused by the influence of bright light exposure in the daytime. The reduced CBT set point also has an effect on skin blood flow. In cold environments, the dermal blood flow has to increase to promote heat loss so that the CBT can be kept at a lower level.
Not only the light intensity is known to be thermoregulating, also the color temperature of the light can be used for thermoregulation. When comparing 3000, 5000 and 7500 K lighting, the increase in rectal temperature just after hot bathing (40° C.) is greatest under bathroom lighting of 3000 K and the higher value was maintained after bathing. This conforms to the observation that light of a higher color temperature results in a lower set point of the CBT. When an individual is bathing, the dermal blood flow has to be low, so that the CBT increase due to heat absorption from the bath is minimal. When the CBT set point decreases, the dermal blood flow further decreases in an attempt to minimize heating of the body core. However, upon leaving the hot water, the individual's dermal blood flow quickly rises. This enables an individual to get rid of the excess heat, thus allowing the CBT to decrease to its set point. A lower CBT set point will increase the dermal blood flow after the bath and will reduce the individual's drop in skin temperature after exiting the water.
To conclude, it can be said that the intensity and the color temperature of the lighting have a direct influence on thermoregulation of a vertebrate being exposed to the lighting. Scientific results indicate that the set point of the core body temperature decreases with increasing intensity and color temperature.
International patent application having publication number WO 2008/120127 generally discloses an interaction system and user interface for mimicking and controlling natural daylight such as by changing attributes of artificial light throughout the day or other time periods, for example, in response to manipulating an input device, such as a knob, a slider, a pointer and/or selectable dials having indicators. In more detail, WO 2008/120127, which is assigned to the present assignee, discloses an interactive lighting control system that includes a user interface operationally coupled to a processor. The processor is also coupled to a memory and is configured to receive user inputs from a user interface and to control at least one light source in accordance with the user input (received from the user interface) and/or upon execution of predetermined programs or light scripts stored in the memory. The light scripts include instructions to control the light sources to provide predetermined static and/or dynamically changing illumination as a function of one or various factors, such as time of day, day of year, season, weather, etc., by changing light attributes provided from the various light sources, such as intensity (i.e., dimming function), color, hue, saturation, direction and the like. Thus, the system of WO 2008/120127 facilitates natural daylight mimicking, and does not tackle the above discussed issues related to thermoregulation.
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