The tiredness that results from orgasm has nothing to do with testosterone levels. It is due to the fact that in order for someone to cum they have to reach a 'peak' in which their heart rate reaches close to the maximum, their respiration increases, and they generally have an increase in muscular tension throughout their entire body. Sleep is the exact opposite - slow heart rate, slow respiration, relaxed muscles. When your vitals all drop, the body naturally begins to feel sleepy and relaxed. It's bio-feedback.
For the same reason, having a hot shower or bath can make people sleepy (if they choose to get comfy and lay down afterwards) because when they get out their body temperature will drop fairly rapidly, even though it's only by a small degree. Your body senses these signals and releases certain chemicals, and those complex chemical compounds enable you to sleep more easily. The light from a television or computer is the opposite - it stimulates the brain in a way similar to daylight, suppressing melatonin and making it more difficult to get a good night's sleep right after.
Sex leads to an increase in testosterone. This primarily comes from the gonads (of those who still have them) but can also come in very small amounts from the pituitary. For those of us on hormones, the difference will be so small that we won't feel it. What we will feel after sex are the neurotransmitters like dopamine (excitement and reward), serotonin (pleasure), and opiods (pain relief and euphoria).
What the source you heard from probably intended to say was that the sperm levels would return to normal within a couple of hours.
