In Ray Bradbury's Fahrenheit 451, Montag returns home to find Mildred in bed:
The breath coming from the nostrils was so faint it stirred only the farthest fringes of life [...] Her face was a snow-covered island upon which clouds might pass their moving shadows, but she felt no shadow. There was only the singing of the thimble-wasps in her tamped-shut ears, and her eyes all glass, and breath going in and out, softly, faintly, in and out of her nostrils, and her not caring whether it came or went, went or came.
On the floor Montag sees the bottle of sleeping pills that had held 30 pills that morning, but now sits empty on the floor.
One can make assumptions with regard to Mildred's overdose. The next morning she says she has no idea what Montag is speaking about when he approaches her at breakfast. She has no recollection of taking the pills and asks what reason she would have to do so. With Mildred's mentally anesthetized condition—brought on by the TV walls in the parlor that she watches for hours on end and the seashell earbuds that speak to her each night—one could assume that she is satisfied with life and that she simply made a mistake. However, there are elements in the novel that point to a subconscious desire to address the terrible emptiness and lack of purpose within her.
First, consider Montag's reaction to Clarisse's comments and ideas.
When Montag meets Clarisse, he thinks she is strange. She notices simple things about the world to which he (and most like him) have grown unable to recognizes. This reflects Mildred's condition as well. When Montag and Clarisse speak for the first time, he sees himself in her eyes, which are lit like candles, holding his image in them. He remembers, as a child, losing the electricity:
...his mother had found and lit a last candle and there had been a brief hour of rediscovery, of such illumination that space lost its vast dimensions and drew comfortably around them...mother and son...transformed, hoping that the power might not come again too soon...
The theme of illumination is central to the story. When Montag considers what Clarisse says ("There's dew on the grass in the morning," and "Are you happy?"), it changes (transforms) how he sees the world and his place in it. Clarisse tells Montag "You never stop to think what I've ask you." More simplified, it seems Montag never stops to think. He has been programmed by his society as has Mildred: take life for its surface value and never think at all; conform to society and be happy in it. For a long while Montag was that way, but he begins to change.
However, Mildred has accepted a life that moves forward without question, meaning or purpose. Her actions demonstrate her conscious desire to leave everything in her life just as it is.
Upon reflection, Montag discovers that he is not happy. He never realized this before. While he has someone who challenges him to think (Clarisse), and he is (surprisingly) open to these ideas that make him examine his existence, his actions and his beliefs, Mildred has no one like Clarisse to open her eyes to the world except Montag. When he tries to get Mildred to think for herself, she resists every attempt her husband makes. For example, when he asks how they met, Mildred is uncaring and annoyed.
Montag's possession of books upsets Mildred. His questions upset her. Her fear that he might lose his job distresses her—because her only wish is to get another TV wall so that she can be fully surrounded by the hype the government feeds its citizens to keep them oppressed in silence, numb and/or empty to emotional response, and bathed in calm acceptance.
Mildred spends each day ingesting social doctrine that is meaningless and well-disguised to be just that. Mildred seems happen enough on the surface, without any desire to see the world as it truly is. She consciously battles against illumination. However, her subconscious seems much more aware of her feelings of dissatisfaction than her conscious self is willing to recognize.
So while Mildred's overdose might have been a mistake, it seems more likely that some part of her subconscious is terribly unfulfilled with a shallow existence that leaves her feeling barren and purposeless. Change is unacceptable to her, and so—unaware—Mildred takes the pills to guarantee a permanent detachment against feelings of hollowness that reach into the core of her psyche.