Date of Award

Spring 1-1-2014

Document Type


Degree Name

Doctor of Philosophy (PhD)


Integrative Physiology

First Advisor

Kenneth P. Wright

Second Advisor

Robert S. Mazzeo

Third Advisor

Monique K. LeBourgeois

Fourth Advisor

Rodger Kram

Fifth Advisor

Edward L. Melanson


Demands of modern society force many work operations into the late night when the intrinsic circadian timing system is promoting sleep. Overnight shiftwork is associated with increased risk for adverse metabolic health, accidents and injury, and sleep disruption. Uncovering potential physiological mechanisms by which working and eating at adverse circadian times contribute to metabolic dysregulation are vital to the development of treatment strategies. Furthermore, understanding the time course of cognitive performance decrements during shiftwork and mechanisms by which caffeine improves alertness and disrupts sleep are important for developing evidence-based fatigue management approaches. Therefore, the aims of this dissertation were to: 1) determine fundamental changes in physiology that would promote a state of unwanted weight gain in response to food consumed during simulated overnight shiftwork; 2) determine how cognitive performance changes during extended wakefulness on the transition to the first night shift and during subsequent night shifts; and 3) determine if early morning caffeine administration influences the distal-proximal skin temperature gradient and improves alertness while subsequently disrupting daytime recovery sleep.

The results indicate: 1) working during the night increased total daily energy expenditure (EE) on the transition to the first nightshift day and decreased total daily EE on the second and third nightshifts, decreased EE during daytime sleep episodes, decreased EE in response to a late dinner meal, increased total daily fat utilization on the first and second nightshifts and reduced carbohydrate and protein utilization on the second nightshift, and decreased subjective hunger despite concurrent decreases in satiety hormones; 2) working during the night increased sleepiness and decreased cognitive performance which predominately did not change across subsequent nightshifts; and 3) caffeine administered 5h prior to daytime recovery sleep significantly decreased the distal-proximal skin temperature gradient, increased alertness, and disrupted daytime recovery sleep.

These findings suggest that reduced energy expenditure during nightshift work may represent a contributing mechanism by which working and eating during the night increases the risk of weight gain and obesity. Furthermore, results from this dissertation expand our knowledge on cognitive performance decrements across subsequent night shifts and physiological mechanisms by which caffeine improves alertness and disturbs sleep.