Multiparameter Sleep Monitoring Using a Depth Camera - Biomedical Engineering Systems and Technologies - page 293

position, and it is 0.0652 in side-lying position. Therefore, the detection threshold of
sleep position is set to the median value: 0.01. While the ratio is larger than the thre-
shold value, the sleep position is defined as the supine position. Otherwise the sleep
position is defined as the side-lying position, as shown in Eq. 4. We can observe that
the standard deviation of the body distance is bigger in side-lying position than others.
It is because the highest torso points are different for female and male. However, our
method can also distinguish the sleep position accurately no matter the gender.
D
D

(3)

0.01 (4)

3.3 Measurement Limitations

There are some measurement limitations in this system. First, according to the law of
rectilinear propagation of light, the depth value cannot be detected while the IR pat-
terns are blocked by objects. From the experiment results, we found that the most
common problem is that the hand would block some of the depth IR patterns while
side-lying. It might affect the accuracy of torso detection. Second, the breathing am-
plitude of torso movement would be decreased with the increase of the thickness of
quilt. According to our test, the average breathing amplitude of the torso movement
with no quilt is 0.5 cm and it is 0.35 cm while sleeping with a thin quilt. The thickness
of the thin quilt in our test is 0.6 cm. However, while sleeping with a thick quilt, such
as thick silk-padding quilts, the system might not accurately detect the torso move-
ment caused by breathing exercise.

4 Experiments

Two experiments were conducted to evaluate the measurement accuracy of head/torso
detection, sleep position, body movement, and breath measurement of this system.
First experiment was mainly designed to evaluate the measurement reliability for
different users. Second experiment was designed to evaluate the measurement accura-
cy in realistic overnight-sleep condition.

4.1 Experiment I: Sleeping Simulation

Experimental Design. Eight participants volunteered to participate in this experiment
(five males and three females). The average age is 33.8 years old (SD = 17.6), including
two sixty-year old participants, five young participants (25~30 years old), and a ten-year
old participant. The body mass index (BMI) of them is in the range between
18.6~29.75. In this experiment, participants were asked to lie down on the pillow, and a
breathing sensor, RIP [28], was used to record the breathing conditions as the ground
truth. During the experimental procedure, they were asked to change the sleep position
every fifteen breathing cycles. The procedure of this experiment is in the sequence of
supine, lying on the right side, supine, lying on the left side, supine, and lying on the