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Now we in the West spend 90 per cent of our time inside, it’s time to pay real attention to the fact that daylight has a powerful influence on health.

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Words by Ruth Kelly Waskett

Let's start by stating an obvious but not always observed fact: with few exceptions, the primary purpose of a building is a place for people to live, work and play.

When it comes to lighting design, it is surprisingly easy to forget that the success of a building is heavily dependent on whether or not its occupants are happy and comfortable. An occupant can be anyone in the building: a visitor who spends an hour there, or an employee who spends eight hours or more in the space.

We know that, when inside buildings, people generally prefer to position themselves near a window, although scientific explanations for this are thin on the ground.

There are a number of theories about why people prefer to be near a window:

• People like to feel connected with what is going on outside, in terms of weather, time of day and activity.
• People enjoy higher levels of daylight because of the benefits to their vision and ability to see colour accurately.
• People find the natural variance of daylight stimulating.

That we appear to prefer daylight is not surprising. We humans have evolved to function within the 24-hour light dark cycle of the earth, and the seasons and their associated day-lengths can have a significant impact on our physical and mental health. A literature review published this year (Daylight and health: a review of the evidence and consequences for the built environment), reveals a diverse range of health impacts that have been studied recently in the quest to find out more about how light affects the human organism.

Coelux skylight: The closest anyone has got to reproducing sunlight

As well as the more widely known aspects, such as sleep quality and seasonal affective disorder (SAD), there were studies looking at the role of light in conditions such as ADHD, autism, breast cancer, onset of puberty and even the link between birth weight and day length at the time of birth.

Our understanding of the effect of light on human physiology and psychology has increased greatly since the turn of the century. We now know that there are receptors within the human eye whose sole purpose is not to see but to trigger various responses within the body, such as the regulation of various hormone levels in the body. One of these is the sleep hormone, melatonin, which is suppressed by exposure to high levels of daylight and naturally increases in the evening, making us feel sleepy.

We recognise that our natural body clock is slightly longer than 24 hours, and that the act of 'resetting' the body clock by exposure to high levels of daylight each morning is key to maintaining a healthy sleep-wake balance. There has been some discussion in the press recently about how light-emitting devices such as tablets and smartphones in the evening might affect our body clock negatively by effectively delaying this natural process.

The spectrum of light is important; we know that light with a strong blue component is particularly effective at suppressing melatonin. Apps such as F.lux promise to counteract this effect by varying the spectrum of light emitted from the screen throughout the day and evening, so that the blue content is reduced at night.

For employers evidence is mounting that providing good access to daylight helps keep workers happy, healthy and productive. For example, one recent study found that employees with a window in their workplace registered, on average 47 minutes a night more sleep. Numerous other studies allude to links between access to daylight, decreased levels of absenteeism and higher productivity.

So how should this affect our approach to building design? We need to bear in mind that glazing acts as a filter to daylight, changing its spectral content. The benefits of daylight already mentioned are gained when the daylight is in its pure state, and the full spectrum of light can be received. For example, the important blue content of daylight (towards the UV end of the visible spectrum) in the morning, needed to suppress melatonin production, is typically filtered out by glazing materials.

We need architectural elements that enable occupants to access unfiltered daylight, and this could be achieved in a number of ways. For example, by the use of glazing materials that allow more of the full spectrum of light to pass through (not necessarily all of the time), by openings in facades, or by features such as covered balconies or verandas that make it easier for occupants to go outside in a range of weather conditions during their working day, even just for short breaks.

Yet it is not simply a case of the more, the better. Uncontrolled ingress of sunlight can cause overheating and therefore discomfort. High daylight levels can cause glare, leading to eyestrain and headaches. In some conditions, such as migraine, high daylight levels are to be actively avoided. This is the major caveat of the 'daylight is healthy' message: in buildings, people must be able to control daylight ingress to suit their needs.

Heathrow T2 (daylight analysis by Hoare Lea): People in the developed world now spend around 90 per cent of their time inside buildings, and daylight is key to human health

If adequate control is not provided, people will find other ways of getting what they need - even if it means sabotaging well-intentioned design features such as sophisticated control systems that do not allow manual override.

The simplest way of providing control is to install blinds on windows. One major drawback of this is that people often leave blinds in the same position for very long periods of time, usually to deal with a worst-case condition that arises at a particular time of day. This means they lose all the benefits of having large windows.

Many highly glazed buildings suffer from this problem, and there have been several efforts to address it. The way forward may lie with smart-glazing materials, such as electrochromic glazing, which allows control without the need to completely break the connection between indoors and outdoors.

One question that has preoccupied many lighting designers and product manufacturers is: can full-spectrum daylight be simulated using artificial sources? There is some evidence that artificial sources whose spectral content is engineered to mimic daylight through the day can affect alertness and mood.

There are several situations where daylight is purposely excluded because of the need to create very specific lighting effects, such as in the leisure sector. Perhaps the use of spectrally enhanced sources should be considered more in these cases, particularly for the benefit of occupants who spend long periods of time in the space, such as workers. Recent product innovations, such as the Coelux rooflight, are clearly aimed at simulating daylight in an interior that does not have direct access to daylight. Visually, it is very convincing, though it's not clear whether the spectral content has been designed to mimic that of real daylight.

Despite ongoing technological development, the ability to truly replicate daylight using artificial sources is unlikely. The fact remains that people in the developed world now spend around 90 per cent of their time inside buildings, and daylight is key to human health. If we want our buildings to work well, then this is something we simply cannot afford to ignore.