Sentence:  When an organism is <WSD>disentrained</WSD> -- that is, when an organism is unable to make contact with the temporal frame of reference provided by the relevant zeitgeber (in this case, the alternating cycle of day and night), such a disentrained organism will operate on the basis of the intrinsic properties of its internal biological clock. 
Meaning of the word marked with <WSD> tags:  To disrupt an organism's circadian rhythm so that it is not aligned with its environment.