The rule in Tomas's sex life is that he can't be in love with anyone.
If he chose one woman to love, then all his mistresses would be jealous, because they were no longer all on an equal plane.
So Tomas's solution is to get Tereza her own apartment, so that none of his other mistresses would know that a woman was spending nights with him ("spending the night together was the corpus delicti of love" (1.6.2)).
Since his divorce Tomas has never fallen asleep with another woman. After sex he always wants to be alone.
That's why he was so surprised at himself for falling asleep with Tereza.
He was even more surprised to find that this made him happy. From then on, the point of making love was more about getting to fall asleep together after. Tereza always held on to some part of Tomas as she slept (like his hand, ankle, pajamas, etc.).
One night he tries to get out of bed while she was half-asleep. She followed him down the stairs of his apartment, took him by the hand, and led him back to bed.
Tomas then concludes that sex and sleeping together are completely different. And love is known only in the latter.