O.C. 231: That all a person's intents be l'shem shamayim (for the sake of Heaven)

"If it is not possible for a person to study [Torah] without an afternoon nap, he should sleep (Rama: And when he wakes up it is not necessary to say the blessing "Eloqai Neshama". And there are those who say that prior to sleeping he should recite "V'hehi noam") provided that he does not lengthen his nap since during the day it is prohibited to sleep more than a "cat nap" [literally "horse sleep" but...]...And even with this little bit his intent should not be in order to benefit his body, but rather to strengthen it for serving God, may He be blessed. And likewise with every benefit in this world he should not intend for his own pleasure but for the service of his Creator, may He be blessed, as it is written, "In all your ways know Him", and our Sages said, "All your actions should be l'shem Shamayim (for the sake of Heaven). Even permitted activities such as eating and drinking [kosher food], walking, sitting, and rising, or marital relations, or conversation, or any bodily need should only be for serving his Creator or something which results in His service.

Even if one is thirsty or hungry, if he eats or drinks to benefit himself this is not praiseworthy, rather he should have the intent that the food and drink give him nourishment in order to serve his Creator.

And likewise one who sits in the place of the upright, stands in the place of the righteous, and walks in the advice of the pure, if he does so to benefit himself and to fulfill his wants and desires, this is not praiseworthy, but rather he should do so l'shem shamayim (for the sake of heaven).

And so too with reclining, it is not necessary to say that at the time he is able to engage in learning Torah and doing mitzvos he should not indulge in sleep to enjoy himself, rather even when the time has arrived and he needs to sleep to rest from being tired, if he does so for the enjoyment of his body this is not praiseworthy, rather he should intend to give sleep to his eyes and rest to his body for the sake of his health, so that his thought will not be torn from Torah from sleep deprivation.

And likewise with marital relations, even at the times required by Torah, if he does so to fulfill his desires or to benefit his body, this is embarrassing. And even if he intends to have children who will be able to assist or to full his place, this is not praiseworthy. Rather he should intend to have children who will serve his Creator, or intend to fulfill his marital obligations like a person fulfilling an obligation.

And likewise with speech, even to speak words of wisdom, one's intend should be to serve his Creator or a matter which results in His service.

The principle of the matter, a person is obligated to place his eyes and his heart on his ways and weigh them on the scales of his intellect, and when he sees something which brings about the service of his Creator, may He be praised, he should do so, and if not he should not do so, and a person who does so serves his Creator continually.

Shulchon Oruch, Orach Chaim 231