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• Effects of photoperiod history on immune responses to intermediate day lengths in Siberian hamsters (Phodopus sungorus). 
• Melatonin regulates energy balance and attenuates fever in Siberian hamsters. 
• Peripubertal immune challenges attenuate reproductive development in male Siberian hamsters (Phodopus sungorus). 
• Photoperiod affects the expression of sex and species differences in leukocyte number and leukocyte trafficking in congeneric hamsters. 
• Photoperiod alters the time course of brain cyclooxygenase-2 expression in Siberian hamsters. 
• Photoperiod controls the induction, retention, and retrieval of antigen-specific immunological memory. 
• Photoperiod influences the effects of exercise and food restriction on an antigen-specific immune response in Siberian hamsters. 
• Photoperiodic adjustments in immune function protect Siberian hamsters from lethal endotoxemia. 
• Sex differences in photoperiodic and stress-induced enhancement of immune function in Siberian hamsters. 
• Sex steroid hormones enhance immune function in male and female Siberian hamsters. 
• Short day lengths attenuate the symptoms of infection in Siberian hamsters. 
• Short day lengths enhance skin immune responses in gonadectomised Siberian hamsters.