Light is formally defined as optical radiation between 380 and 780 nanometers (nm) that provides visual sensation in humans.1 The human eye has photoreceptors that convert radiant energy into neural signals for processing by the brain, a phenomenon called phototransduction. Until recently, four types of photoreceptors had been identified: rods and short, middle, and long wavelength cones. Rods allow us to see at night and cones allow us to see during the day, discriminate details, and discern colors. The neurophysiology and neuroanatomy of the human visual system is largely understood and all lighting technologies, standards, measurement devices, and applications have been based solely on that understanding.
In the past few decades much work has been done to understand the non-visual effects of light on human health and wellbeing. Light entering the human eye is transmitted to several parts of the brain that are different than those projecting to the visual centers. In 2002, researchers discovered the intrinsically photosensitive retinal ganglion cell (ipRGCs), a novel photoreceptor type in the retina.2 The ipRGCs are central to an important “non-visual” response to light by the retina, most notably the regulation of circadian rhythms.
The world rotates around its axis and, as a result, all creatures exposed to daylight on earth experience 24-hour cycles of light and dark. Biological rhythms are self-sustaining oscillations with a set of species-specific characteristics, including amplitude, phase, and period.3 Living organisms have adapted to this daily rotation of the earth by developing biological rhythms that repeat at approximately 24 hours. These circadian rhythms are generated endogenously (internal to the body) and are constantly aligned with the environment by zeitgebers (time givers), factors exogenous or external to the body. In mammals, circadian rhythms are regulated by an internal biological clock (pacemaker) located in the suprachiasmatic nuclei (SCN) of the hypothalamus of the brain.3 The SCN is a self-sustaining oscillator that maintains its daily activities for weeks when isolated and cultured. The SCN in humans has a natural period that is slightly greater than 24 hours and environmental cues can reset and synchronize the SCN daily, ensuring that the organism’s behavioral and physiological rhythms are in synchrony with the daily rhythms in its environment. The light-dark cycle is the main synchronizer of the SCN to the solar day4 and reaches the SCN via the retinohypothalamic tract (RHT). Although the circadian system shares receptors and neurons in the retina with the visual system, the retinal ganglion cells exiting the eye for the visual centers are different than those exiting the eye for the circadian system. Light as well as dark play an important role in regulating our circadian rhythms, and the timing of light-dark cycles are profoundly important for many of our behaviors and our wellbeing.
Light can have both an acute and a phase-shifting effect on the internal clock. Acute effects are seen shortly after light exposure. For example, it takes about 5-10 minutes for light to suppress the production of nocturnal melatonin, a hormone produced at night and under condition of darkness.5 Acute effects also disappear soon after a light stimulus is removed; after about 30-45 minutes in the dark, nocturnal melatonin returns to a normal level. Phase shifting effects are seen a few hours or days after light exposure. Light applied during the early part of the night will delay the clock (for example, bed and waking times will occur later than in the previous cycle) and light applied very late at night or in the early morning will advance the clock (bed and waking times will occur earlier than in the previous cycle). If light is applied before minimum core body temperature, it will delay the clock while light applied after minimum core body temperature will advance the clock. Minimum core body temperature typically occurs about 2-3 hours before one wakes up naturally (i.e., without an alarm clock).6
Lighting characteristics affecting the circadian system, as measured by acute melatonin suppression and phase shifting of dim light melatonin onset (DLMO), are different than those affecting visibility. Rods, cones, and the intrinsically photosensitive retinal ganglion cells (ipRGCs) participate in circadian phototransduction, which is how the retina converts light signals into neural signals for the biological clock. It is now known that lower levels of light, less than those originally demonstrated in the 1980s, can acutely decrease melatonin concentration and affect the timing of melatonin onset and offset; however, light levels needed to affect melatonin are still higher than those needed to affect vision. For example, a warm color (correlated color temperature of 2700 K or lower) nightlight delivering 1 lux at the cornea will allow one to safely navigate in a space at night, but it will not suppress the hormone melatonin. Humans are blue sky detectors—the peak sensitivity for acute melatonin suppression and phase shifting of DLMO is close to 460 nanometers (nm). The effects of light on the circadian system vary over the course of the 24-hour day. Morning light, given after the trough of core body temperature that typically occurs in the second half of the night, will advance the timing of sleep in the following cycle, while evening light, given prior to the trough of core body temperature, will delay the timing of sleep. Photic history, or the amount of light received during the previous day determines the effectiveness of light on acute melatonin suppression and on the phase shifting of the timing of DLMO. For maximum results of light therapy, it is also important to accurately measure light exposures over the 24-hour day, as opposed to taking just a “snapshot” measurement of light exposure at one certain place and time. The circadian system seems to keep track of light exposure, and therefore, knowing an individual’s light exposure history over the past 24 hours can help determine the best light prescription for the next 24 hours.
The effects of light on the circadian system are not trivial and should always be taken into consideration when designing lighting in a space. Upsetting the daily pattern of circadian light and dark affects performance and wellbeing, as well as, it seems, our basic health. The therapeutic value of circadian 4 light (and dark) has been demonstrated in laboratory and field studies. Light with the appropriate characteristics can reduce symptoms of seasonal affective disorder, increase sleep efficiency of older adults including those with Alzheimer’s disease,7-10 improve circadian entrainment of premature infants,11 and increase alertness and wellbeing of night-shift workers.12-14 And it is not just about short-wavelength (blue) light. Recent research shows that long-wavelength (red) light, which does not affect melatonin levels at night, can increase objective and subjective measures of alertness as well as certain types of performance. 15-17 Light during the day can also affect alertness and certain types of performance. Figueiro and colleagues showed that about 200 lux at the eye of a warm white light (2650 K) and 260 lux of a 640-nm light increased alertness and certain types of performance during the post-lunch dip hours.18, 19 It has even been suggested that some forms of cancer and cardiovascular disease may be linked to the disruption of the normal circadian light-dark pattern. The impact of light on the general population is less clear. More recent studies suggest that daylight in office buildings20 can increase sleep in workers, but further investigation about the effect of light on the general population is warranted.