Multiparameter Sleep Monitoring Using a Depth Camera - Biomedical Engineering Systems and Technologies - page 292

Body Movement. The body movement is defined as the sum of head movement and
torso movement. The HR t indicates the average depth value of the head ROI in time t ,
and the TR t indicates the average depth value of the head ROI in time t . Then, the
absolute difference value between two adjacent images frames could be calculated by
Eq. 2. M t indicates the movement value of the user in time t .
M |HR HR ||TR TR | (2)
Sleep Position. In this system, two main sleep positions (supine position and side-
lying position) can be recognized. After the head and torso are detected, the highest
point of head ROI and torso ROI can be found. Then, the ratio of the highest head
point to torso point is calculated. Fig. 4 shows the highest point of head ROI (blue
dot) and torso ROI (red dot) in the side-lying position and supine position.

Fig. 4. Sleep Positions. Red dot indicates the highest point of torso ROI, and blue dot indicates
the highest point of head ROI

1300

Head Body

1250

1237

1200 1234 1235

1224

1150

1263

1270

1100

1153 1147

1050

1000

Supine Side Lying Supine Side Lying

Sleep without quilt Sleep with a thin quilt

Fig. 5. The distance between the depth camera and the highest point of head ROI and torso ROI
in two positions (supine and side-lying) and two conditions (sleep with no quilt and sleep with
a thin quilt)

Next, the sleep position can be classified according to the ratio defined in equation
3. In order to find out the threshold to classify the sleep position, an experiment was
conducted to record five participant's (two females and three males) highest points of
head ROI and torso ROI in two sleep positions (side-lying and supine) and two condi-
tions (sleep with no quilt and sleep with a thin quilt) (Fig. 5). The results revealed that
the distance of the highest head point does not change significantly in different sleep
positions and conditions. However, the distance of the highest torso point changed
significantly in different sleep positions. The average ratio is -0.02633 in supine