Human helping behavior is a ubiquitous and fundamental aspect of Homo sapiens, playing a crucial role in the development and maintenance of modern civilizations. From international aid packages to individual acts of charity, helping is pervasive across societal strata and contributes significantly to a global "helping economy." While various factors have been linked to helping behavior—evolutionary pressures (kin selection, reciprocal altruism), cultural norms, socioeconomic factors, and personality traits (empathy)—the role of sleep has remained largely unexplored. This study investigates the hypothesis that sleep loss significantly impacts human helping behavior. The rationale for this hypothesis stems from two key observations: firstly, insufficient sleep impairs emotional processing, affecting emotion recognition and expression while increasing basic emotional reactivity, factors linked to antisocial behavior; and secondly, sleep deprivation consistently decreases activity and disrupts connectivity within the social cognition brain network, a neural system crucial for prosocial behavior. This network includes regions like the medial prefrontal cortex (mPFC), temporal-parietal junction (TPJ), and precuneus, which are activated when considering the needs and perspectives of others and making the choice to help. Damage to these regions can lead to "acquired sociopathy," characterized by a loss of empathy and reduced helping behavior. Therefore, this research aims to determine whether a lack of sleep impairs human helping at multiple levels: neural, individual, group, and societal.

Existing literature extensively documents the diverse factors influencing prosocial behavior. Evolutionary perspectives highlight kin selection and reciprocal altruism as mechanisms favoring helping close relatives and those likely to reciprocate. Cultural differences also significantly impact helping tendencies, with collectivistic cultures often exhibiting higher rates of helping than individualistic cultures. Socioeconomic factors, such as urban density, also correlate with helping rates, with rural areas generally showing higher levels of helping. Personality traits, particularly empathy, are consistently linked to prosocial behavior; individuals high in empathy are more likely to help others. However, the existing literature lacks a comprehensive investigation into the role of sleep in modulating prosocial behavior. Although studies have demonstrated the negative impacts of sleep deprivation on various cognitive and emotional processes, the direct causal link between sleep loss and helping behavior has not been explicitly examined.

This study employs a multi-level approach to examine the impact of sleep loss on human helping behavior. Three separate studies are conducted, each targeting a different level of analysis: individual, group, and societal. **Study 1 (In-laboratory Experiment):** This study uses a counterbalanced, crossover design with 24 healthy adults. Participants undergo two sessions: one after a full night's sleep and one after 24 hours of sleep deprivation. In each session, participants complete a 40-item helping behavior questionnaire (adapted from the Self-Report Altruism Scale and the Prosocial Personality Battery), assessing their willingness to help in various scenarios involving both strangers and familiar individuals. Mood is measured using the PANAS scale, and an incentivized effort task is included to control for potential effort-related effects. During the fMRI scan, participants perform a social judgment task designed to assess social cognition network activity while making personality judgments about individuals depicted on cards. The fMRI data is analyzed focusing on the a priori defined social cognition network using SPM12. **Study 2 (Online Micro-longitudinal Study):** This study uses a micro-longitudinal design with 136 participants completing daily sleep diaries and a shortened version of the helping behavior questionnaire for four consecutive days. This examines the relationship between natural fluctuations in sleep quality (sleep efficiency) and sleep quantity (duration) and daily changes in the desire to help others. Linear mixed-effects models account for between-person and within-person effects, controlling for age, sex, survey version, mood, and prior helping behavior. **Study 3 (Online Donations Database):** This study examines real-world altruistic helping behavior using data from over 3 million charitable donations made through the DonorsChoose website between 2001 and 2016 in the US. The analysis focuses on donation amounts during the week following the annual transition to Daylight Saving Time (DST) in spring, comparing it to donation amounts in the surrounding weeks and months, while accounting for day of the week, month, year, and holidays. Similar analysis is performed around the transition back to standard time (ST) as a control condition. Multiple regression models are employed to test for the effect of the DST transition on donation amounts, and control for several factors, like seasonality, and the number of donations.

The findings across the three studies consistently demonstrate a negative association between sleep and helping behavior across different scales: **Study 1:** One night of sleep deprivation significantly reduced the desire to help others compared to a sleep-rested condition (N = 23, P = 0.011, d = 1.054). This effect was consistent across 78% of participants and remained significant even after controlling for mood changes and effort. fMRI analysis showed reduced activity in the social cognition network following sleep deprivation (P = 0.02, d = 0.7), and the degree of network impairment predicted the reduction in helping behavior (R = 0.42, P = 0.046). Importantly, the impact of sleep loss on helping was similar for strangers and familiar individuals. **Study 2:** Night-to-night reductions in sleep efficiency were associated with a decrease in the desire to help the following day, both within and between individuals (within-person effect, β = 0.02, P < 0.05; between-person effect, β = 0.04, P < 0.001). These effects were independent of sleep duration, mood changes, and trait empathy. Even controlling for the previous day's helping behavior, the relationship between sleep efficiency and the subsequent helping desire remained significant. **Study 3:** The week following the transition to Daylight Saving Time was associated with significantly lower donation amounts compared to surrounding weeks (BDST week = -0.09 ± 0.04, P < 0.05). The effect was specific to the spring DST transition, with the autumn ST transition showing no significant impact on donations. This finding remained consistent even after accounting for time available for donations and seasonal effects on donation patterns. In contrast, the transition to ST did not exhibit a significant effect.

This research provides robust evidence supporting the novel hypothesis that sleep loss significantly impairs human helping behavior across multiple levels of social interaction. Study 1 establishes the causal link between sleep deprivation and reduced helping, identifying the social cognition brain network as a key neural mechanism underlying this effect. Study 2 extends these findings by demonstrating that even small, ecologically relevant variations in sleep efficiency under free-living conditions predict daily changes in helping behavior. Study 3 further corroborates this relationship by showing that the societal-level manipulation of Daylight Saving Time leads to a reduction in real-world altruistic helping. The consistency of these findings across different methodologies and scales underscores the substantial impact of sleep on prosocial behavior. The observed effects are not simply explained by changes in mood, effort, or attention, highlighting the specific role of the social cognition network and suggesting a complex interplay between sleep, neurobiological mechanisms, and social behavior. The bidirectional nature of the observed effects – sleep deprivation reducing helping, and adequate sleep restoring it – suggests sleep as a potential target for interventions aiming to promote prosocial behavior.

This study provides strong evidence establishing a crucial, previously unrecognized link between sleep and prosocial helping behavior. Insufficient sleep, whether experimental or naturally occurring, consistently predicts reductions in the desire to help and in the actual act of helping, observed at individual, group, and societal levels. These findings suggest that optimizing sleep may be a critical, modifiable factor for enhancing prosocial behavior and strengthening societal cooperation. Future research could explore the specific neurochemical pathways mediating this relationship and investigate the broader societal implications of sleep deprivation on social cohesion and the "helping economy".

While this study provides compelling evidence for the link between sleep and helping behavior, some limitations should be noted. The samples, while statistically significant, might not fully represent the diversity of the human population. The helping behavior questionnaires, while widely used, might not capture the full spectrum of helping behaviors. In Study 3, reliance on a single online donation platform may limit the generalizability of the findings. Furthermore, while controlling for several confounding variables, it is important to acknowledge the complex interplay of factors that contribute to prosocial behaviour, which this study was not designed to fully elucidate.