When Lady Macbeth counsels her husband in this way, suggesting that he is distraught because he has gone without sleep, her comment assumes a great deal of irony later. In Act IV, it is Lady Macbeth who cannot sleep without light around her (she fears the dark) and when she does fall asleep, she cannot rest. She walks in her sleep, reliving the monstrous deeds for which she and Macbeth are responsible and suffering the horror of them. How ironic that she, the woman who had seemed to be so untouched by the blood they had shed, soon finds that she, too, would "lack the season of all natures." There is a bit of dramatic irony in her comment, also, because the audience knows what she has not recognized, that Macbeth will never sleep well again after killing the king.
An additional bit of irony in her words could be related to King Duncan. Duncan was living an innocent life, sleeping peacefully, when Macbeth murdered him. Since Macbeth and Lady Macbeth had violated Duncan's sleep to take his life, it is fitting that they forfeited their own peaceful sleep in doing so. Ironically, they had believed that killing Duncan in his sleep would be a safe way to give them the throne and great happiness.