Feeling sleepy immediately after orgasm in men (and less often in women) is the result of a specific hormonal mechanism, not a lack of interest in a partner.
The role of prolactin Immediately after ejaculation, levels of prolactin rise sharply—the same hormone that is also involved in sleep regulation. High prolactin is directly linked to sleepiness.
The combination of serotonin and oxytocin The combined action of these hormones calms the nervous system and reduces alertness—according to evolutionary theories, this may have historically served to provide a “rest” phase.
Individual differences Not all men experience this in the same way—initial prolactin levels, age, and overall health all affect the intensity.
How to resolve this in a couple Openly talking about this biological reality (and not perceiving it as indifference to your partner) is often enough to prevent it from becoming a cause for conflict.