All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The wife is obliged to respond to the request of her husband whenever he asks her to bed for sexual intercourse unless she has a sound reason for refusing, like when she is in menses, sick and the like. As for sleeping, this is not a valid reason for her to refuse. Similarly, being exhausted is not a sound reason unless this causes her great hardship.
Therefore, it is very appropriate to discuss this with your husband so that he would not request when it is difficult for you to respond to his wish. Likewise, the husband should not disturb his wife and make her go through difficulties which she cannot bear.
For more details, please refer to Fataawa 84944 and 84980.
Allaah Knows best.