London is currently home to productions of four Samuel Beckett plays. A trilogy – Not I, Footfalls and Rockaby – performed by the extraordinary Lisa Dwan, is at the Royal Court Theatre in advance of a transfer to the Duchess Theatre. Across town, Juliet Stevenson takes on the role of Winnie in Happy Days at the Young Vic Theatre. Both productions are preoccupied with altered states. And, for anyone interested in the health humanities, the concept of the altered state is unavoidable and fascinating.

Not I, the first play in the Royal Court trilogy, plunges its audience into a darkness that is unremittingly absolute. Every glimmer of residual light is extinguished and the effect is devastating. There is nothing between us and ourselves. No distracting or reassuring visual clues that we have a place in the world. All that remains is the blackness. We wait until our eyes adapt – for surely they will adapt – isn’t that what eyes do? Isn’t adaptation the essence of humankind? But they don’t. We don’t. It is inescapable: our altered state.

And then, looming above us in a beam of light, is the mouth. We cannot help but focus. It commands our focus. This disembodied mouth. All that there is in the room is the darkness, the mouth and our thoughts. And so it begins. Words, sounds, glimpses of sentences tumble out of those bright red lips – a life pouring forth, demanding to be heard. At first we can’t make out the words. Some are familiar, but some a nonsense. Is it the speed? Is it the accent? We are concentrating so hard; why can’t we understand? And still the words cascade into the darkness and still we search for meaning. We revert to the comfort of clinical categorisation: this is logorrhea. But how pointless that seems – what is the value of naming but never knowing?

And still the mouth moves and the waves of sound wash over the auditorium. We sense damage. We intuit harm. We no longer need the details – we can feel it. In our altered state, all communication convention is overthrown and we discover it doesn’t matter. If we persist in attending to another, then we will make a connection.

In contrast to the claustrophobic darkness of Not I, the theatre in Happy Days is assaulted by light. It is brutal and unforgiving in its reach. Under the burning brightness, we find Winnie buried up to her breasts. As with the plays in the trilogy, we know nothing of how she came to be here. Beckett is not concerned with causality – it is enough simply to meet a person where and how they are. So, we encounter Winnie, trapped in the earth for reasons we will never understand nor need to understand.

Winnie’s altered state is physically, emotionally and metaphorically concerned with what lies beneath. The audience too, whilst looking only at the top half of her body in an unchanging set, begins to grapple with the unseen and the unexpressed. As Winnie digs deeper into the bag she treasures, her memories, hopes and identity emerge in the form of mundane, and practical objects. Her failed efforts to engage Willie – to share her altered state with another – require her to dig ever deeper into her self. The increasingly brittle humour with which she meets her predicament discomforts us – as altered states so often do.

When Act 2 opens, Winnie’s state is further altered and she is buried up to her neck. All that is visible now is her head. Her hair is disheveled and her teeth rotting. She is herself and yet, so crushingly and irrevocably altered. Her decline is physical and existential. She is in pain. She is unsure whether she is alone because she can no longer turn to see Willie. She can no longer dig deep – and what lies beneath is suffocating her.

As Willie painstakingly crawls his way across the earth, upwards towards Winnie and the gun that rests in front of her, the potential narratives flood our minds. His intentions are ambiguous to the last. But this is not a state that ends. The lights dim and the actors remain frozen on the verge of change. A change we can imagine, but cannot control. An uncertain, but unavoidable, change that mirrors the shifting and complex experiences and perceptions of those altered states that, in other contexts, we call ‘illness’.

Not I, Footfalls and Rockaby is at the Duchess Theatre from 3-15 February 2014

Happy Days is at The Young Vic Theatre until 8 March 2014