Diel rhythms of physiology and behaviour are present in virtually all taxa examined to date. However, few studies have rigorously assessed the adaptive value of physiological and behavioural rhythms in animals. We laid the groundwork for an assessment of the adaptive rhythm hypothesis by assaying diel rhythms of foraging and antipredator behaviour in the trashline orb-weaver Cyclosa turbinata (Araneae: Araneidae). When confronted with a predator stimulus in experimental arenas, C.turbinata showed thanatosis behaviour more frequently and for longer durations during the day. However, assays of antipredator response within webs revealed more complex diel patterns of avoidance behaviours and no pattern of avoidance behaviour duration. A preliminary assay of prey capture behaviour suggested that C.turbinata also shows nocturnal patterns of foraging aggression. A refined foraging experiment, in which we randomized prey stimulus frequency, found that time of day and spider ID strongly predicted the likelihood of showing prey capture behaviour. Only spider ID predicted latency to attack the prey stimulus despite low individual repeatability (r= 0.10). These results support our prediction that C.turbinata modulate foraging aggression over the diel cycle, but we found no evidence of a trade-off between foraging behaviour and predator vigilance. However, overall patterns of vigilance may be masked by diel changes in antipredator strategies that correspond to fluctuations in the relative abundances of predator types.