Multitasking Is Hardest
In The Early Morning
Multitasking seems to come easier for some and is virtually impossible
for others, however new research shows that it is difficult for
all in the late night and early morning.
Previous studies have shown that the time of day greatly affects
human's reaction time (for a review, see Carrier & Monk, 2000).
This performance decrement is constantly found during the night
with its' lowest point in the early morning. This leads to the
assumption that the time of day directly affects the speed of
Daniel Bratzke at the University of Tuebingen wished to take
these studies a bit further and figure out what makes our reaction
time so slow during the wee hours. While many researchers have
studied this, Bratzke focused on one of the three stages of human
processing because he argues that measuring overall reaction time
does not allow researchers to separate the effects of three different
processing stages. Bratzke describes, for example, that the stage
models of human performance assume at least three distinct processing
stages: early perceptual, central decisional and late motor. He
writes, "Given this widely accepted view, the question arises
whether time of day affects all processing stages in general or
one or more stages selectively."
There is evidence that the circadian variations in reaction time
are at least partly due to changes in two of the processing stages;
in the early perceptual stage for example, it takes longer to
detect and identify a visual stimulus if you are tired. Likewise,
manual dexterity, grip strength, and tapping, which assess motor
ability, are susceptible to our circadian rhythm, with a low point
in the morning and peak in the late evening.
Bratzke wished to localize the time-of-day effects on processes
that occur during the central decisional stage, such as decision-making.
Along with his colleagues, he hypothesized that reaction time
performance decrements during the night might be associated with
slowing of this central processing stage. They refer to this as
the central-slowing hypothesis.
Bratzke observed six subjects during 28 hours of constant wakefulness.
During this time, the subjects performed various cognitive tasks
every two hours. Bratzke used a dual-task performance test (psychological
refractory period paradigm) to assess the duration of the central
processing stage with two stimuli introduced at different times.
The results provided evidence for a circadian modulation on reaction
time with a constant decrement in the late evening while reaching
it's lowest point in the early morning. Importantly, dual-task
interference increased in the night and early morning as well.
Thus, central processing slowed down during the night, a result
that supports the central-slowing hypothesis.
It is important to note that Bratzke monitored circadian phases
through salivary melatonin concentration and body temperature.
These biological markers of circadian phases corresponded with
the decrement in reaction time, telling us that sleep deprivation
was not the only moderator in the performance decrement.
These results build upon a recent study showing that even the
well-practiced simple task of vehicle braking is subject to dual-task
slowing. When watching the car brake in front of you while processing
another stimulus concurrently the braking response can be markedly
slowed down. The present results suggest that dual-task slowing
as e.g. in this driving situation is influenced by the time of
day with the most pronounced slowing in the early morning. Bratzke
writes, "in addition to sleepiness, a combination of slowed reactions
and impaired central efficiency might contribute to impaired driving
performance and a higher risk for traffic accidents in the early