Human psychology also plays a role with respect to timing of colour temperature variations. Preferred light settings may vary with time of day. Therefore, users should be given the opportunity to adjust colours themselves, preferably when the risk of phase delay or advances is lower.
We also know that light in the early hours after nadir gives stronger effect on rhythm shifts than later in the day. Knowing this, we can set the time for light exposure to achieve the wanted effect. But since people have different chronotypes, the timing has to be set carefully. ”Night owls” have a later wake-up time than early risers or “larks”. The duration of the cycle may also vary from 23 hours for the lark to perhaps 26 hours for the owl. Hence, one may risk feeding the owls with activating light at the wrong side of nadir. For normal office hours, light exposure from 9 am and onwards could therefore be used as a rule of thumb. This will help people to adapt better to less daylight in the winter time.
For workers on night shift over several days, it might be helpful to shift the rhythm 8 hours backward to reduce the sleepiness during night. Using the knowledge above, this can be achieved by exposing the worker to cool white light in the late evening/early night for a couple of hours (depending on the intensity). This will shift the curve backwards over a couple of days.
Cool white light may have a positive effect on subjective alertness and mood. The effect is sudden and will pass minutes after the exposure has ended. These acute effects of bright light exposure on subjective alertness, fatigue and vitality are also independent on time of day. However, the effect of bright light exposure on sustained attention is most significant in the morning.
What about individual differences and preferences?
Not only human chronotypes vary, but also their preferences. There is no one size fits all for tuneable white lighting solutions. What may have excellent phase-shifting effects on one person may have negative effects on others. In class rooms, where the level of individual adaptation of the lighting is more difficult to set, pupils and teachers will receive the same spectrum, intensity and exposure. This calls for even greater care to be taken when planning for these applications. The same applies for open offices, but individual adaptations are still easier to implement in the form of personalised lighting such as task lights, free standing lights or pendants above each work station. Lighting solutions in industrial work stations and patient rooms in hospitals may also be easier to set individually.
Distribution of light
The ganglion cells of the third photoreceptor are most sensitive in the nasal and lower area of the retina. This is due to the eye adapting to natural lighting conditions, because daylight enters the eye from above.
Light coming from angles above 60 ° relative to the horizontal plane and below the horizontal plane has little or no effect on melatonin production. This since most Ganglion cells are placed in the nasal and lower area of the retina. Light coming from the "right" angle must not be perceived as discomfort glare.