  Daylight plays a major role in the wake/sleep cycle in humans. Indeed, the
wake/sleep system stems from biological systems that follow a circadian rhythm
determined by the light/dark alternation. The oscillations can be modeled by
the higher order non-linearity van der Pol -type equation driven by a term that
mimics the light cycle. In this work noise in the illumination is introduced to
investigate its effect on the human circadian cycle. It is found that the
presence of noise is detrimental for the sleep/wake rhythm, except for some
special values for which it may favor regular oscillations. Depending for
system parameters, noise induces regularities, such as stochastic resonance: if
the natural light is turned off, it emerges that there is an optimal value of
intensity noise which most deteriorates the regularity of the cycle, it is the
phenomenon of anti-coherent resonance. Also, the phenomenon of stochastic
resonance occurs: in the presence of the drive of natural light, there is an
optimal noise intensity which improves the evolution of the wake / sleep
system. However, there is a critical value of the noise beyond which the system
becomes chaotic; indeed, for sufficiently high noise levels (how high depends
upon the parameter of the system), the sleep/wake cycle evolves in a random and
unpredictable manner, for whatever parameters of the external light.
