Shulchan Arukh, Orach Chayim 231

That all of one's intentions be for the sake of Heaven. And there is one paragraph in it.
If it is impossible for him to study without sleeping in the afternoon, he should sleep. Gloss: And when he wakes from his sleep, he does not need to recite the blessing, "My Lord, the soul..." (Beit Yosef). And some say that he should read, "May the pleasantness..." before he sleeps (Kol Bo). [And this is] so long as he does not make it long; as it is forbidden to sleep during the day more than the sleep of a horse, which is sixty breaths. And even in this little, his intent should not be for the pleasure of his body, but rather to strengthen his body for the service of God, may He be blessed. And likewise with anything that is pleasant for him in this world, his intent should not be for his pleasure, but rather for the service of the Creator, may He be blessed; as it is written (Proverbs 3:6), "In all of your ways, know Him." And the Sages said, "Let all of your actions be for the sake of Heaven." As even optional things – such as eating, drinking, walking, sitting, getting up, sexual relations and conversation – all of them should be for the service of your Creator or for something that enables serving Him. For even if one was thirsty or hungry – if he ate or drank for his pleasure, it is not praiseworthy. Rather he should intend to eat and drink for his sustenance, to serve his Creator. And likewise even to sit in the council of the righteous, to stand in the place of the saintly and to go in the path of the straight – if he does it for his own pleasure and to fulfill his desire, it is not praiseworthy; only when he does it for the sake of Heaven. And likewise with laying down: There is no need to say that at a time when he is able to be in involved with Torah study or with commandments, he should not entice himself with sleep to enjoy himself. Rather even at a time when he is weary and needs to sleep in order to rest from his weariness – if he does [so] for the pleasure of his body, it is not praiseworthy. Rather he should intend to give sleep to his eyes and rest to his body for the requirements of health, so that his mind not get jumbled during Torah study on account of sleep deprivation. And likewise with sexual relations: Even at the time [prescribed] by the Torah, if he does [it] for the pleasure of his body, it is disgraceful. And even if his intent is in order that he will [produce] children that will serve him and take his place, it is not praiseworthy. Rather he should intend that he will [produce] children for the service of his Creator; or he should intend to fulfill the commandment of the [prescribed] time, like someone who is paying his debt. And likewise with conversation: Even to recount words of wisdom, his intent must be for the service of the Creator or for something that leads to serving Him. The principle of the matter [is that] one is obligated to put his eyes and his heart to his ways and evaluate all of his actions on the scales of his intellect: So when he sees something that will bring him to serving the Creator, may He be blessed, he should do it. But if not, he should not do it. And one who acts like this will be serving his Creator constantly.