When a person is in a deep sleep, will they feel a kiss?

The ability for a slumbering person to notice an outside stimulus, such as a kiss, while sleeping depends on what sleep stage the person is in during the time of the stimulation. Even in the deepest stages of sleep, some stimuli may be noticeable to the sleeping person.

The first stage of sleep is one of relaxation and drowsing, and the second stage of sleep is light, so outside stimuli are most noticeable during these sleep stages. The third and fourth sleep stages are deeper, so outside stimuli are less likely to rouse someone who is in these stages of sleep.