1. In 1729, an experiment was conducted on a sensitive plant that demonstrated plants have endogenous circadian rhythms that continue in the absence of daylight cues. 2. In humans, melatonin production and core body temperature are two biological processes used to determine the endogenous circadian rhythm. Bright light exposure at different points in the circadian cycle can cause phase delays or advances in body temperature rhythms. 3. For night shift workers, using bright light therapy aimed at entraining the lowest core body temperature to occur during daylight hours can help promote alertness at night and sleepiness during the day. However, maintaining a strict light/dark schedule is important for effective entrainment.

1. Bright Light & The Night Shift Tech Spring 2001
By Regina Patrick, RPSGT, Associate Editor
An early experiment on biological rhythms was done by Jean Jacque d’Ortous deMairan in 1729
on the sensitive plant (Mimosa pudica) whose leaves open in the day & close at night. He put the plant
in total darkness & found that the plant leaves’ behavior continued despite not having the external cues
of daylight & darkness. Although this experiment demonstrated that the plants had an endogenous
circadian rhythm. Plants had a circadian rhythm which consisted of a “tension phase” & a “relaxation
phase” but some felt that these phases were driven by the advent of sunlight & nightfall rather than
internally driven.
In humans, 2 biological processes used to determine the endogenous rhythm are melatonin
production & the core body temperature.
1. Dim light situations (e.g. nighttime or a dark room) prompts the production of melatonin which
then brings about sleep. With the advent of daylight, melatonin production is suppressed
resulting in wakefulness. Bright light with an intensity of 2500 lux could completely suppress
nocturnal melatonin production in humans (500 lux is approximately the intensity of normal
room light).
2. The lowest core body temperature (Tmin) normally occurs about 1-2 hours before one awakens
for the day. As the day progresses, body temperature rises then at dusk it starts to fall as
melatonin production begins. Bright light given before Tmin, induces a delay of the temperature
rhythm phases while bright light given after Tmin induces an advancement of temperature
rhythm phases. This response of phase delay or phase advance only occurs around Tmin. At Tmax,
no response of phase delay or phase advance occurs with bright light exposure.

2. Short pulse time, as little as 3 hours, can effectively entrain the endogenous rhythm3
. Tmin, the
sleepiest part of the temperature rhythm, normally occurs during the night. Bright light pulses, however,
can entrain it to daytime hours. For a night shift worker, a daytime Tmin is ideal. With Tmin occurring in
the daytime, the biological clock is set so that sleep will occur during the day & wakefulness at night.
Experiments have shown that subjects feel more alert when entrainment is accomplished by
causing phase delays rather than phase advances. Therefore, an ideal schedule to entrain Tmin to
daytime hours begins by giving a pulse (typically 3-5 hours) before Tmin on the first night. This will cause
a phase delay so that Tmin occurs about 2 hours later on N2. Subsequently, the light pulse is given 2
hours later on N2 & then successively 2 hours later each following day until Tmin falls in the desired
(daytime) hours. Once Tmin is entrained to the desired time, it becomes important to maintain a strict
light/dark schedule with the use of bright light exposure on awakening to halt any continued delays of
Tmin4
. If Tmin were to continue to shift daily, problems with sleepiness would remain.
Although not a widespread practice, light boxes are used in many workplaces to provide the
needed intensity for bright light therapy during the night shift. Many sleep centers have success with
using light boxes for night shift technicians. For example, David Duhon, MD of the Sleep Disorders
Center of Central Texas, Austin, Texas remarks: “We had a box type in the control room for years, & it
seemed to work well for the techs inclined to use it. I had an employee years ago with narcolepsy & she
claimed that using the bright light was helpful in fending off sleep attacks.” However, some centers have
less success. Technologists use light boxes for awhile, then quit when alertness does not seem improved
or they quit for other reasons. As explained below, failure of bright light therapy may be a reflection of
an ineffective light/dark schedule, inappropriate light, or the arrangement of light boxes used in the
control room rather than an inherent problem with the technician. (“It just doesn’t work for me.”). If
these problems are addressed, then technologists who currently continue to suffer from sleepiness with
bright light therapy could instead start benefiting from it.
A technologist may feel no difference in their alertness
“Bright light during the night may help your techs stay awake during the night, but not
necessarily to have a better sleep during the day. In my opinion, it will be even more important to tell
your techs to keep their bedroom as dark as possible during their daytime sleep.” As reflected in this
statement by researcher Marie Dumond, Ph.D., of Hospital Sacre-Coeur in Montreal, Canada
maintaining a strict light/dark schedule is very important. Because it such a powerful zeitgeber, light at
the wrong time can prevent entrainment by causing unintentional phase advancements or phase delays.
For example, if sunlight on the ride home is occurring after Tmin, the sunlight — being brighter — will
override the phase delay intended by bright light treatment during the night shift & cause instead a
phase advance.
Simulated night-shift studies have proven that on the ride home it is important to limit the
sunlight reaching the eyes3
. In such studies, subjects used welder’s goggles which filtered out slightly
more than 99% of incoming light thereby allowing for the dark phase to continue in broad daylight.
Although this is great for entrainment, welder’s goggles are dangerous to use while driving. Sunglasses
with top & side shields are a more practical alternative. With side shields, sunglasses filter out 93% of
incoming light which will still allow entrainment but the phase shift response is not as strong as with
welder’s goggles. Without side shields, most sunglasses can filter out up to 90% of light but 10% or more
of light transmittance to the eyes is too much to allow for entrainment.
Sleepiness may remain if light boxes are not arranged optimally so that a worker receives the
full intensity of light from the light box. In one study, photometers — representing a worker’s eyes —
were used to measure the intensity of illumination received by a worker at a work station at various
distances from the light box5
. Pointed directly at the center of the light box & 1 foot away from the box,
the photometer measured 20% of illumination of the rated output of the light box. As the photometer

3. was moved 6 feet away from the light box, the illumination received by the photometer dropped to 5%
to 10% of the rated output of the light box. When measuring the illumination that would be received by
a worker looking downward (as if for reading), the photometer measured approximately 4% of the rated
output of the light box at 1 foot away from the box. As the photometer was moved 6 feet away from the
box, this dropped to 1% to 2%. This study illustrates how illumination received by a worker drastically
drops as a worker looks away from or moves away from the light box. To receive the most benefit from
a light box, a worker would have to sit very close & look directly at the box. If there is only one box
providing bright light, this would limit technologist movement. An alternative, so that light covers more
of a worker’s visual field, is to use several light boxes around the control room or have greater diffusion
of the light.
The light is too bright
Most studies have been done with bright light frequently in the range of 5000 lux or more. For
many, this is bothersome to the eyes or may cause other problems such as being able to see a computer
monitor screen. Medium intensity light in the range of 1070 lux to 1400 lux has been found to cause
phase delays in body temperature comparable to high intensity (5200 lux to 7500 lux) light6
. Since
entrainment is still possible, medium intensity light could be an alternative for technicians who are
having problems with more intense light.
Techs find the box too bulky
Although boxes can be large (approximately 2 feet by 1 foot by 5 inches), companies now make
light boxes in various sizes for a variety of situations: traveling, sitting in a control room at a work
station, etc. “I now have a desk lamp style light & it seems to work just fine. Takes up less space than the
box,” lauds Duhon.
Technologists may not consistently work the night shift
Using bright light to entrain may still be feasible for some workers who rotate shifts slowly.
Because the schedule listed earlier takes several days to entrain Tmin to a desired time, the worker
whose schedule changes slowly (weeks or months) will benefit more from bright light therapy than
someone whose schedule changes more quickly. When worker schedule shifts frequently, the body itself
cannot change that rapidly even if light pulses are used.
References
1. Campbell SS, et al., “Light treatment for sleep disorders: consensus report . 1.Chronobiology of
seminal studies in humans”, Journal of Biological Rhythms, 10(2), Jun 1995, 105-109
2. Lewy AJ, Wehr TA, Goodwin FK, Newsome DA, Markey SP, “Light supresses melatonin secretion in
humans”, Science 210:1267-1269, 1980
3. Eastman, CI, et al, “Light treatment for sleep disorders: consensus report. VI. Shift work”, Journal of
Biological Rhythms, 10(2):157-164, June 1995
4. Eastman CI & Martin SK, “How to use light & dark to produce circadian adaptation to night shift
work”, Annals of Medicine, 31:87-98, 1999
5. Drew Dawson & Scott S. Campbell, “Bright light treatment: are we keeping our subjects in the
dark?”, Sleep, 13(3):267-271, 1990
6. Stacia K. Martin & Charmane I. Eastman, “Medium-intensity light produces circadian rhythm
adaptation to simulated night-shift work”, Sleep, 21(2):154-165, 1998