In School and Out by Oliver Optic

For six or seven nights following the expedition to the watermelon patch of Mr. Batterman, Richard Grant did not "walk in his sleep. " The parental solicitude of his father prompted him to set a watch for several nights; and Mr. Presby, who was still anxious to pursue his scientific investigations, slept with one eye open, that he might be in readiness to avail himself of the reappearance of the phenomenon.

The philosopher's hint that sleep-walkers are sensitive to any allusion to their infirmity, had prevented him and Mr. Grant from informing the subject of their precautions of the steps they had taken to observe his movements, and Richard was entirely unconscious that vigilant eyes were upon him while he slept, or while he ought to sleep.