Lighting systems have historically been designed for visual performance and comfort, but science tells us light plays a major role in synchronizing the human circadian system. The growing understanding of the relationship between light and circadian health is giving rise to a new lighting trend: circadian lighting.
Hospitals present a particularly interesting application. This is an environment specifically designed to promote health where managers are concerned both with the well-being of workers and their patients. The 2012 Commercial Building Energy Consumption Study, conducted by the U.S. Department of Energy, estimated there were 10,000 inpatient healthcare buildings in the United States, representing about 2.4 billion square feet.
The human body regulates biological functions based on 24-hour cycles known as circadian rhythms—one being timing the release of the hormone melatonin, which tells the body when it is time to sleep.
Disruption of the body’s internal clock also affects the timing of these bodily functions and, therefore, can lead to poor nighttime sleep as well as mental and physical health problems.
For millions of years, the patterns of light and dark produced by Earth’s rotation regulated circadian rhythms. Today, the average person spends most of their time indoors where light is predominantly provided by electric sources. Traditional lighting design focused on vision, not accounting for light’s nonvisual effects on the brain.
As our understanding of circadian health grows, building owners are becoming increasingly interested in it. The key factors are light levels (the amount of light falling on the eye’s photoreceptors throughout the day through vertical illuminance), spectrum (the light wavelength, commonly associated with its color appearance), timing (when the eye receives light), duration (the time span of light exposure), and history (the cumulative occurrence of light and dark patterns received at the eyes over time).
In a traditionally designed interior lighting system, the amount of light delivered is calculated on horizontal surfaces such as desktops. Light-level recommendations are based on vision, and achievable light levels are limited by energy code restrictions on wattage. Spectrum is typically fixed (likely cool white), and occupants receive the same light level all day, unless daylight is available.
In contrast, because light received at the eye stimulates the circadian system, a lighting system that supports the circadian system would provide significant vertical illuminance (light on walls, task lighting, etc.) to ensure a sufficient quantity of light (20–40 foot-candles) falling on the eye’s photoreceptors. Controls would gradually lower the light levels and shift color appearance from cool to warm throughout the day to entrain the occupants’ circadian systems. Ideally, the occupants would have access to daylight.
The result should be a circadian stimulus (CS) of 0.3+ for at least one hour in the early part of the day. CS is a metric developed by the Lighting Research Center (LRC). The threshold for circadian response is 0.1, and the saturation point is 0.7. Designers should be careful about relying on correlated color temperature (CCT) as a circadian metric, because it can be misleading. They should instead use dedicated metrics, such as CS. Achieving this entails a change in design thinking and requires flexibility.
Circadian lighting and hospitals
Mariana Figueiro, professor and director of the LRC at Rensselaer Polytechnic Institute in Troy, N.Y., is one of the leading researchers in the country on circadian lighting. The LRC recently expanded LightingPatternsforHealthyBuildings.org to include designs for healthcare facilities based on the CS metric and the online tool it developed to evaluate design effectiveness. It has conducted a significant amount of research into the relationship between light and health, most recently involving an installation of circadian lighting at Mount Sinai Health System in New York City, which showed promising early results.
“Various writers have likened the hospital to a city in microcosm, which poses a formidable challenge to lighting design because, in this case, the ‘city’ is all under one roof,” Figueiro said.
In some respects, designing lighting for vision and CS in a hospital is the same as any other application, such as an office. The Illuminating Engineering Society recommends daytime light levels of 50 foot-candles on the workplane for general tasks in both applications. The lighting should deliver a CS greater than 0.3 during the day, particularly early in the day.
Unlike most offices, hospitals work 24/7, and their populations include both workers and patients. Some of these workers put in long hours, including night and rotating shifts, and exposing patients 24/7 to the same interior lighting disrupts their circadian rhythms. Therefore, the lighting in healthcare facilities must be flexible to meet diverse needs.
Night lighting can be particularly challenging because it must allow patients to sleep while staff work.
“It’s a delicate balance that invites creative solutions,” Figueiro said. “Here, it would be helpful for designers to think of lighting in layers, tailoring the exposures to the space, the person occupying it, and what is required for what the person is doing, whether working or sleeping and healing.”
High CS during the day and low CS in the evening should be the basis of the design. Additional layers of light can accommodate critical visual tasks and increase alertness in nightshift workers.
“Circadian-friendliness isn’t the least bit antithetical to functionality,” Figueiro said. “At one time, we might have been compelled to make tradeoffs due to deficiencies in technology and imagination, but any gap that existed between the two capacities is closing quickly, and it’s the job of imagination to come out on top.”
A prime example is the Swedish Healthy Home, a joint project between the LRC, Lund University and the Swedish Energy Agency. The research team developed an integrated network of light and activity sensors that tracks and records personal 24-hour light exposures and activity, and then it develops a personalized lighting prescription.
“The geometric growth of the internet of things points to a time when we will be able to deliver an around-the-clock lighting prescription to people outside the home,” Figueiro said. “The same technology could be employed today, or at least in the very near future, throughout the ‘city’ of a modern healthcare building.”
The healthcare designs on the Lighting Patterns for Healthy Buildings website cover three distinctive applications: patient rooms, a nurse’s station and a neonatal intensive care unit (NICU).
The NICU requires careful light zoning to serve two unique populations. One is premature infants, who at a certain point in their development can benefit from receiving at least two hours of CS in the morning. The other is nursing staff, who require lighting for circadian health and alertness throughout their shift.
Nurse’s stations also require flexibility. These spaces support day and night shifts, which have different lighting needs. During the day, the lighting should be designed for CS to promote alertness. At night, it should avoid circadian disruption for patients (by providing low CS) while promoting alertness and providing sufficient light for staff to complete work.
Let’s examine a typical nurse’s station illuminated by 2-by-4 troffers fitted with 3,500K (cool white), 32-watt (W) T8 lamps providing 50 foot-candles on the workplane. This design produces a CS of 0.2 around the clock, which is too low during the morning and too high at night.
An alternative would be to install dimmable and color-tunable linear LED overhead luminaires and wallwashers. During the day shift, the horizontal illuminance level could start at 50 foot-candles and increase in the afternoon before reducing again in the evening. The wallwashers, which produce substantial vertical illuminance, would dim to 75 percent in the evening to reduce horizontal and vertical light levels. CCT would shift from 5,000K to 4,000K during the day for the overhead luminaires, while the wallwashers would produce a saturated blue light that drops to 5,000K or stays blue in the afternoon and drops to 4,000K in the evening. The result is a CS of 0.5 in the morning, 0.3 in the afternoon and 0.2 in the evening.
During the night shift, the overhead luminaires remain at full output to produce a horizontal illuminance of 33–35 foot-candles, while the wallwashers dim to 50 percent at certain times of the night, resulting in vertical illuminance of 12–15 foot-candles. Overhead CCT drops to 3,500K, while the wallwashers also shift to 3,500K and later a saturated red light, and then either remain red until the day shift begins or return to 3,500K. The resulting CS is 0.1 throughout the night.
Patient room lighting should provide high CS during the day and low CS in the evening. Night lighting should support patient sleep while accommodating visitors and staff tasks.
As another example, consider a single-patient room illuminated by recessed 2-by-4 troffers fitted with 3,500K, 32W T8 lamps and a wall-mounted, direct/indirect task light over the bed. With output set at 26 foot-candles, this system provides a sufficient CS of 0.3 for the morning, but there is little flexibility to reduce it when needed.
An alternative would be to install recessed linear LED luminaires; a wall-mounted, direct/indirect task light; a few downlights at the entrance and perimeter; and a recessed linear LED wallwasher. The linear light, task light and wallwasher dim throughout the day until stopping at 25 percent in the evening. The downlights stay on at full most of the day but dim to 50 percent in the evening. All lights shift from 5,000K in the morning to 4,000K at midday and 3,500K in the late afternoon and evening, while the wallwasher could emit a saturated blue light in the morning or remain on a tunable-white setting throughout the day. The result is CS that starts the morning at 0.3, drops to 0.2 at midday, and declines to 0.1 in the late afternoon and evening.
Lighting for healthy buildings
In a short time, the lighting community went from a basic understanding of lighting and health to equipment, metrics and design templates suitable for many applications, including hospitals.
Figueiro said circadian lighting is actionable and likely to trend further as resources and research strengthen over time. She encouraged electrical contractors to familiarize themselves with new products and techniques.
“The technology now exists to develop more individualized solutions for healthcare applications,” she said. “Think beyond the ceiling.”