Instrumentation & Measurement Magazine 24-2 - 31

before bed could shift their body's natural sleep response, resulting in sleep deprivation and morning fatigue from not
enough restful sleep [11]. Sound can affect sleep through nocturnal awakenings, while sleeping in an acoustically isolated
room has shown benefits of increased deep sleep and latency
of REM sleep [12]. In addition, temperature may affect their
sleep by creating discomfort for too high or low temperatures
especially during summer and winter [13]. Sleeping in a colder
environment can have effects on cardiac properties such as
heart rate and heart rate variability as the body tries to move
blood to generate heat, while hotter environments can increase
wakefulness and reduce deeper sleep [13]. Furthermore, high
humidity levels prevent sweat from evaporating properly,
and therefore the body is not able to cool down quickly [13].
Bed sheets and pajamas are common for temperature regulation during sleep with the area under the bed sheets creating a
microclimate [13], which insulates the body temperature and
maintains a comfortable amount of heat for the sleeper.

Wearable and Non-Wearable Sleep
Tracking IoT Sensors
Actigraphy
Actigraphy estimates sleep and wake states by analyzing activity from body movements, typically measured from the wrist
[5]. The body movements are measured and collected through
an inertial sensor that comprises of an accelerometer, a gyroscope, and a magnetometer. The latter two sensors provide
accurate estimates of orientation and external accelerations
(e.g., gravity) via Kalman filtering for more comprehensive
body movement analyses [14]. These sensors are worn by the
person overnight to capture physical activity and analyzed
the following day when they wake up. Initially, the coordinate components of the accelerometer are normalized to obtain
the magnitude for analyzing the activity. Then, there are three
common methods for converting the raw analog acceleration values into usable digital activity counts for analysis: time
above threshold, zero-crossing method, and digital integration [5]. Time above threshold counts the amount of time that
the acceleration signal is above a threshold to determine activity events [5]. Zero-crossing method counts the number of
times the acceleration signal crosses zero to be used as the activity events [5]. Finally, the digital integration method calculates
the area under the curve to measure the acceleration activity for
each epoch [5]. Fig. 1 shows a sample normalized acceleration
signal that can be used with one of these methods to obtain a
digital activity signal to be used for sleep/wake analysis.
To estimate the sleep and wake states from actigraphy, a
weighted sums analysis developed by Cole et al. referred to
as the Cole-Kripke equation is used to estimate sleep/wake
states from the actigraphy device accelerations [6]:
	

 X A  X2 A4  X3 A3  X4 A2  X5 A1  
D  P  1 5
	(1)
X6 A0  X7 A1  X8 A2



where the A values are the 30 second movement activity and
the X values are the weights to emphasize epochs. Activity A0
April 2021	

Fig. 1. Normalized raw acceleration for sleep/wake scoring.

is the current epoch being scored with all the future and past
epochs added together. This is to evaluate if there is additional
excessive movement around the current epoch to confirm the
sleep or wake state based on how active a person is. The P
value is the scaling factor to adjust the equation with the standard deviation of all participants. The Cole-Kripke equation
estimates sleep and wake when it exceeds a threshold for D >
1 being wake and D < 1 being sleep [6]. The idea is that a person would be awake when there is excessive movement and
sleeping during no movement. However, this leads to overestimating sleep states as the algorithm will score an awake
person who is not moving as asleep because only the movement activities are being measured in actigraphy. Therefore,
rescoring rules have been proposed in the literature to observe
instances of consecutive activity and rescore short inactivity
as wake rather than having many oscillating stages [6]. Actigraphy is a convenient method to analyze daily activity and
estimate sleep states over long periods of time. In addition,
many consumer devices include accelerometers already so users can easily use these devices to begin tracking sleep states in
a non-intrusive manner [2].

Ballistocardiography
BCG measures cardiac properties from body vibrations produced by the recoil effect of the blood flow through the body
and heart [15]. ECG signals measure the heart pulse through
the QRS-complex, and the R spikes identify the heart beats
[16]. BCG signals are analogous to this, but the notation is referred to as IJK to distinguish the two separate methods [16].
Since the BCG device is typically placed on the mattress and
not attached directly to the participant like an ECG, the signals must be filtered to remove all movement artifacts from the
cardiac signals. This can be done with a low-pass infinite impulse response filter to the beat to beat signals obtained from
the BCG device [16]:
	

y t  

1  k   y  t  1  k  x  t 	(2)

IEEE Instrumentation & Measurement Magazine	31



Instrumentation & Measurement Magazine 24-2

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