After the blistering run of Blasted last month, the Crucible Theatre continues (and sadly concludes) its Sarah Kane season this week with alternate nights of Crave and 4.48 Psychosis. Both productions are of astounding power: two brutal segments of 40-odd minutes of broken brilliant words building to an almost insurmountable truth. They show how far Kane was moving away from traditional story structure and characters, but how much she was able to achieve with this form: splintered narratives and fractured dialogue, cutting across each other, drifting in and out of collective consciousness; litanies, lists, stories in fragments, never building to a singular whole yet impossible to get out of your head. We do not come to know these characters as, say, we might feel we come to know Ian or Cate in Blasted, but with the slices of their lives served up to us to intensely we know them in a very intimate sense. It is like a stranger on the bus giving you the epiphany of their life story which cannot be concluded before it is your stop.

Broadly, Crave is about love and 4.48 Psychosis about insanity: they sit well as a pair. 4.48 Psychosis has long been beloved of student fringe productions everywhere, and probably suffers not a little damage from that. As a cruelly revelatory swansong, performed only posthumously after Kane’s suicide, it is a work of enormous weight and consequence. It requires a dexterous delicacy not to make the serenades of enlightened mental breakdown seem either trite or vainglorious. It is not hard to see why young theatre companies, craving edginess, make it their badge of honour.

But Charlotte Gwinner’s direction is lucid and tight; the cast is perfect. It is hard to emphasise just how difficult it is to pull off theatre like this with real power: protracted text of so much unrelenting insight such as this is hard to play truthfully, without overtly ‘acting’. It needs intensity but it also needs restraint. The trio of Pearl Chanda, Rakie Ayola and Tom Mothersdale manage it brilliantly. Mothersdale especially has a gift for injecting lines of unremitting bleakness with a black humour that reminds us Kane’s writing has as much blunt wit as it does gravity; and it’s welcome respite, in all this, to laugh at times. It is rare that you actually have to sit for a few minutes after the lights have gone up to get your breath back. Consider, also, that for press night they’d already done a matinee of Crave the same day, and their emotional stamina deserves all the more praise. This is not lengthy theatre but it is dense and difficult and it takes no prisoners.

Crave stands out to me as the more poignant piece, and in truth, the more tragic. As devastating – and prevalent – as mental illness is, everyone everywhere has known the loneliness of craving love, losing love, being in love, or just being. It is a painful eulogy to the brittle destructiveness of the human condition. Written in a relentless round system, with actors passing lines across one another, sometimes carrying the baton, sometimes changing tack entirely, it is again a style that seems easy but is very hard to do well. It is flawless. The cast are even more in sync for this one, joined by a fourth, Christopher Fulford – who as the wheedling, obsessive, paedophilic A is just about one of the most disturbing creations I’ve had the displeasure of meeting onstage.

The Crucible Studio moulds perfectly to the intensity of these short pieces that are at once static, yet move us far beyond what we thought possible. After the meticulous ingenuity of the hotel room conjured in Blasted, the Studio now is stripped bare, returned to its three-tiered stark box setting in the round. It is a bald uninviting stage: for 4.48 Psychosis, the middle section is lowered, to act as a sort of consulting room, while the outer rim begins to look more and more like a battlement as the actors stalk across it, trying to escape the pen that locks them in their heads. Crave begins in the blackness with the whisper of voices, rising to the frenzied winding of cassette tape. It is a brief and innocuous touch but it sets the mood brilliantly. The lighting for both productions is sparse but pitch perfect: significant enough to reflect each changing mood but too subtle to stand out.

Sarah Kane was a seminal voice who is often adulated but rarely given the stage she deserves. It is enormously important that the Crucible has put on not just a wildcard production of one or two works, but a whole season. Rehearsed readings of Skin, Phaedra’s Love, and Cleansed were also part of the season. I know of several writers who came up from southern climes to see the productions. It is supposed, perhaps more readily than we realise, that nothing dynamic happens outside of London – or at the very least that that is the place to which all good productions eventually devolve. But both the quality and selection of writers and work given a platform at the Crucible consistently marks it out as a bastion of theatre, not just at the so-called ‘regional’ level, but national too.

Crave and 4.48 Psychosis are playing at the Crucible Theatre, Sheffield until 21 March. For more information and tickets, see the Sheffield Theatres website.