2007, Piran, Slovenia

Working environment
EFFECTS OF LIGHT ON TIME SENSE FOR SHORT INTERVALS
Tetsuo Katsuura, Takumi Yasuda and Xinqin Jin
Chiba University, Chiba, Japan
Contact person: katsu@faculty.chiba-u.jp
INTRODUCTION
It has been reported that the perception of short-interval timing, or the time sense, is affected
by several psychological and physiological factors; e.g., age (Espinosa-Fernández et al., 2003;
Gunstad et al., 2006), gender (Espinosa-Fernández et al., 2003), time of day (Kuriyama et al.,
2003, 2005), and the menstrual cycle (Morita et al., 2005). However, it has not been
confirmed that the light environment could affect the time sense for short intervals.
Most higher organisms, including humans, have two endogenous timekeeping systems: a
circadian clock and an interval timing clock (Morell, 1996). The circadian clock, located in
the suprachiasmatic nuclei, is reset every day by sunlight and provides the time of day. The
interval timing clock, located in structures of the brain called striato-cortical loops, gauges the
passing of seconds or minutes and tells creatures how to time their movements when hunting
or evading predators (Morell, 1996). It has been postulated that the activity level of the central
nervous system could influence this latter time sense. Light can influence the central nervous
system, therefore time sense might be affected by lighting conditions. We measured the time
sense and the central nervous system in several lighting conditions to clarify the influence of
light on time sense.
METHODS
Nine healthy young adult volunteers (seven males and two females) were exposed to red light
or blue light radiating from color fluorescent lamps in the ceiling. The illuminance was kept at
310 lx at the subject’s eye level. The wavelength of peak emission from the red-light lamp
was 612 nm, and that from the blue-light lamp was 436 nm.
The subject sat quietly on a chair for 20 minutes, and then was asked to produce two 90-s
time intervals and one 180-s time interval by pressing a stopwatch button. The display of the
stopwatch was covered by a seal to mask the digits of time. The subject started the stopwatch
at the cue of the experimenter and stopped it when he/she thought that 90 s or 180 s had
passed. The subject received no feedback at any point during the experiment.
Then the subject conducted an oddball task to extract P300 event-related potentials. Standard
(1000 Hz, 70 dB SPL) and target (2000 Hz, 70 dB SPL) auditory stimuli were presented
through a headphone. The target stimuli occurred randomly with a 0.2 probability. The
subject was instructed to react to the target stimulus as quickly as possible by pressing a key.
EEG activities at Fz, Cz, and Pz were amplified by a telemetric amplifier. The band pass filter
was set at 0.53-30 Hz, and the EEG signals were digitized at a sampling rate of 1000 Hz for
600 ms with a pre-stimulus baseline of 200 ms. The P300 waveforms were averaged from at
least 25 artifact-free recordings. The P300 amplitude was measured relative to the pre-
stimulus baseline and was defined as the largest positive-going peak occurring within the
latency between 250 and 500 ms. After accomplishing the oddball task, visual analogue scale
(VAS) was used to assess feelings of sleepiness.
Another group of 20 healthy young male subjects were exposed to four lighting conditions
(illuminance/color temperature: 200 lx/3000 K, 200 lx/7500 K, 1500 lx/3000 K, and 1500
lx/7500 K). They were asked to produce four 180-s time intervals in each lighting condition in
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