2.1.3. Position identification and criteria
Sleep positions were classified into four types for further analysis:
the supine position, left lateral position, right lateral position,
and prone position. Because position changes continuously during
sleep, the sleep positions were included in the analysis when they
could be clearly identified as one of these four positions. For
example, if a participant began to sleep in the supine position, the
supine position would begin at that moment and would end when
the next identifiable position was observed, which also marked the
beginning of the second position. The duration and frequency of the
four positions were recorded.