The longer you stay awake, the greater the toll fatigue takes on your ability to function. One place where your performance degrades is in your cognitive functions. Here are a few interesting findings:

• If you stay up for 17 hours, your hand-eye coordination will be similar to how you perform after drinking enough alcohol to bring your blood alcohol level up to 0.05%[1]
• If you stay up for 18 hours, your ability to problem solve, maintain vigilance and communicate will decrease by 30%, add another 30 hours of wakefulness and you will decrease another 30%[2]
• Fatigue will also make your brain slow and sluggish. This means that it will take you longer to react to important information[3]
• Fatigue can also make it difficult for you to process visual information from the peripheral retina[4] and to subsequently react to it.

References

[1] Dawson, D., & Reid, K. (1997). Fatigue, alcohol and performance impairment. Nature, 388, 235.

[2]  Angus, R., Pigeau, R., & Heslegrave, R. (1992). Sustained operation studies: From the field to the laboratory. In C. Stampi (Ed.) Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep. Boston: Birkhäuser.

[3] See for examples:

• Belenky, G., Wesensten, N., Thorne, D., Thomas M., Sing, H., Redmond, D., Russo, M., & Balkin, T. (2003). Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: A sleep dose-response study. Journal of Sleep Research, 12(1), 1-12.
• Dinges, D. (1992). Probing the limits of functional capability: The effects of sleep loss on short –duration tasks. In Broughton, R. (Ed.) Sleep, Arousal, and Performance. Boston: Birkhäuser.
• Galy, E., Mélan, C., & Cariou, M. (2008). Investigation of task performance variations according to task requirements and alertness across the 24-h day in shift workers. Ergonomics, 51(9), 1338-1351.
• Van Dongen, H., Maislin, G., Mullington J., & Dinges, D., (2003). The cumulative cost of additional wakefulness: Dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep, 26(2), 117-126.
• Wylie, C. & Mackie, R. (1988). Stress and sonar operator performance: Enhancing target detection performance by means of signal injection and feedback. Goleta, CA: Essex Corporation, Human Factors Research Division.

[4] Haworth, N., Heffernan, C., & Horne, E. (1989). Fatigue in truck accidents. Report No. 3. Victoria: Monash University Accident Research Centre.