It is not uncommon for cats to exhibit sleepiness or lethargy after receiving a flea treatment. The active ingredients in certain topical spot-on treatments can have a systemic effect on your cat's body. These ingredients are designed to target and kill fleas and ticks by affecting their nervous systems. However, they can also have a mild sedative effect on cats, leading to drowsiness or lethargy. The degree of sleepiness can vary from cat to cat, and some cats may be more sensitive to the effects of the treatment than others. Additionally, the application process itself can be stressful for cats, and stress can contribute to feelings of tiredness. The handling, scent, or sensation of the treatment on their skin may temporarily disrupt their normal routine and cause them to seek extra rest.