Table 3 Summary of depression-related empirical evidence

Mood

(n = 72)

Participants were prompted to complete questionnaires via alarms or ‘beeps’ at semi-random intervals over a range of consecutive days using handheld devices (e.g., palmtops), smartphones and online surveys

Higher negative affect (NA), lower positive affect (PA), lower interest/pleasure in activities, less emotion regulation strategies, and higher variability in affect correlated with depression severity. Additionally, depressed participants tended to overestimate prospective NA indicating a predisposition to have a pessimistic life perspective. Finally, depressed participants reported a larger decrease in dysphoria, sadness, and anxiety when exposed to a positive event compared to healthy participants

While most reviewed studies reported that depressed participants showed higher fluctuations of NA, 2 studies using clinical samples (Gruber et al. [72]; Heininga et al.) [79], and 1 study using non-clinical sample (Pe et al.) [151] observed no significant differences across depressed and non-depressed participants in PA mean values and variability

Psychomotor Activity

(n = 33)

Active data collection via questionnaires via mobile apps, handheld devices, and online questionnaires. Passive data collection via wearable technology, GPS, accelerometer/actigraph, Wi-Fi location, smartphone usage, and typing metadata

Lower levels of physical activity were associated with increased levels of negative affect, depressive feelings, and anhedonia (e.g., reduced ability to enjoy pleasurable activities)

Two studies employing non-clinical samples and GPS-derived data found no significant associations between these variables (Chow et al. [30]; Melcher et al.) [128]

Social Functioning

(n = 21)

Active data collection via self-reported questionnaires, and passive data collection via smartphone embedded audio features, and phone call/SMS frequency

Increased levels of depression severity associated with preference for being alone, increased social distance, reduced closeness with other individuals, increased interpersonal stress, reduced speech duration, and reduced phone call and SMS frequency

Depression severity showed an association with reliance on social expression such that higher reliance on social expression of feelings (i.e., anger) predicted a decrease in depression severity over time (Chue et al.) [31]

Moukaddam et al. [136] used a clinical sample and found no correlations between depression levels and social interaction (SMS and phone call length and frequency)

Sleep Quality

(n = 16)

Assessment of sleep quality involved self-reported questionnaires, accelerometer inferences (e.g., total steps during bedtime), GPS-derived data, actigraphy, smartphone embedded light sensors (e.g., increased light exposure during bedtime), smartphone use (screen on/off), sound features (e.g., ambient silence), and heart rate (assessed via wearable technology)

Most studies detected associations in variability of sleep quality and depression severity. Specifically, studies observed depression scores to be positively correlated with delayed sleep phase, sleep disturbance during weeknights, poor sleep quality, sleep variability, insomnia, and increased exposure to light during bedtime (Ben-Zeev et al. [12]; Di Matteo et al., [49]; Difrancesco et al., [52]; Elovainio et al., [56]; Hung et al., [88]; Kaufmann et al., [100]; Kim et al., [105]; Melcher et al.,) [128]

Two studies (1 clinical and 1 non-clinical sample) did not find significant correlations between self-reports of sleep duration and depression (Difrancesco et al., [52]; Hamilton et al.) [76]. Additionally, 2 studies using non-clinical samples found no significant associations in depression levels and sleep quality assessed via actigraph (Melcher et al.) [128] and self-reports (Hamilton et al.) [76]

Cognitive Style

(n = 19)

Assessment of relationships between depression severity and cognitive style (including trait rumination, self-criticism, reassurance seeking, etc.) involved self-reported questionnaires collected via smartphones and digital devices

Studies observed positive associations between depression severity and fluctuations in self-assessment, reassurance seeking, emotional dependency, self-criticism, trait rumination, experiential avoidance, expressive suppression, and ‘should’ PA (i.e., the pressing feeling that they should experience positive affect)

Cognitive Performance

(n = 6)

Assessment of cognitive performance involved questionnaires (i.e., accordance to statements such as “I have trouble concentrating right now” (Brown et al.) [24], time spent and frequency of errors completing questionnaires (Hung et al.) [88], typing kinematic performance (Vesel et al. [187]; Zulueta et al.) [200], and cognitive tasks (Cormack et al. [37]; Stasak et al.) [170]

Studies observed that higher depression severity resulted in higher thought impairment, fewer clear thoughts, more concentration problems, and reduced cognitive performance

Hung et al. [88] observed that depressed participants did not take longer or make more mistakes than controls in completing questionnaires about mood and quality of sleep