Environmental resource and risk both constrain grouping patterns, but may be weighted differently between day and night. Primates with high fission-fusion dynamics may change their grouping patterns and behavior in tandem with a shift from resource competition to risk avoidance. Using scan data (January 2013-August 2013), I compared subgroup size, stability, cohesion, behavior, and sleeping tree reuse patterns of black-handed spider monkeys (Ateles geoffroyi) at day and dusk at Runaway Creek Nature Reserve, Belize. At dusk, subgroup size significantly increased, and the monkeys rested and engaged in actively social behavior significantly more than during the day. Individuals clustered into family groups at dusk, increasing their distance from presumed non-relatives. Some sleeping trees were preferentially reused over others. Predation risk may make a larger, more cohesive subgroup optimal at night, but the limited quantity and fixed location of preferentially reused sleeping trees may keep subgroups from dramatically changing.