Juliet Lyon, director of the Prison Reform Trust

Most people would say they know something about prison. They recognise the abiding austere image of the barred cell window or, in contrast, the holiday camp, beloved by the tabloid press, where old lags are reported to laze at taxpayers' expense. Yet, despite our zest for incarceration, few really know what life is like beyond the locked iron gates.

Prisons are our least visible, most beleaguered public service. At the close of the year, 70 of 117 prisons in England and Wales are overcrowded. As prison numbers soared to over 85,000 in 2015, drastic budget cuts saw staff numbers plummet. Serious assaults rose and purposeful activity fell to highest and lowest recorded levels respectively. The facts and figures about the state of our prisons and the state of people in them are carefully laid out and referenced in our Bromley Briefings Prison Factfile, Autumn 2015.

Nothing prepares you though, either for the eerie silence of Wormwood Scrubs as you walk down wide empty corridors past closed workshops and past row after row of metal doors each concealing two or three men in cells designed for one. Or for the chaos and press of the dilapidated transit camp that is Pentonville as it somehow manages the movement of men to and fro to London courts, over 33,000 last year alone. Or for waves of confusion and distress at Holloway as women separated from their children, many for the first time, file into dormitories or detox.

Unlike schools where disruptive pupils can be excluded, hospitals where difficult patients can be refused treatment, prisons must take in all those sent by the courts. In such a controlled, closed environment, prison governors and staff have surprisingly little, or no, control about who is received into their institutions. Longer and harsher sentences, a rash of mandatory and indeterminate terms and misuse as a capacious social service have driven prisons beyond their certified normal accommodation and up to their limits of operational capacity. In November Wandsworth, built to hold 963 men, had to find spaces for 1,592. Leeds built for 669 instead held 1152.

And what of those who can't or won't comply with prison regimes, who break prison rules, who challenge authority, who are deemed too difficult, too vulnerable or too demanding to be managed on the crowded wing or landing? Well they go deeper still into custody and end up in the prison within the prison, in segregation.

The Prison Reform Trust has just published an in-depth research report Deep Custody: Segregation Units and Close Supervision Centres in England and Wales. The report by international expert Sharon Shalev and Kimmett Edgar finds that segregation units and close supervision centres (CSCs) entail social isolation, inactivity, and increased control of prisoners - a combination proven to harm mental health. Despite largely good relations with prison staff, people held in segregation experience impoverished regimes with poor levels of purposeful activity. Many prisoners spent their time doing nothing. One man interviewed simply said: "I sit there with my head in my hands."

An active day should be the norm in segregation units, with a focus on the prisoner's needs and the conduct that resulted in segregation. This would give prisoners something to work on while segregated, making their time there more constructive. It would also help to clarify why the prisoner was segregated, thus contributing to a sense of fairness. International standards in the provision of exercise in the fresh air should be met. Above all segregation, although it may sometimes be necessary, must not be prolonged or indefinite.

Over half of the 50 prisoners interviewed for the study reported three or more mental health problems including anxiety, depression, anger, difficulty in concentration, insomnia, and an increased risk of self-harm. Almost half of the 49 officers interviewed said that they would benefit from more mental health training and that further training should be offered.

Writing in the foreword to the report, Lord Woolf, Chair of the Prison Reform Trust and former Lord Chief Justice, said: "Care must be taken to avoid, as far as is possible, the damage to mental health that exclusion will bring. Equally, care should be taken to avoid the use of segregation as a holding operation for people who should be transferred swiftly and humanely to a secure hospital or psychiatric unit."

Perhaps the most worrying finding of the report was the large numbers of prisoners who had deliberately engineered a move into segregation. Over one-third of the prisoners interviewed who were held in segregation units had done so in order to escape violence and indiscipline on prison wings or to raise concerns regarding their treatment and conditions. This, the report says, is an "important barometer of conditions on normal location" and the prison service "should target efforts to improve treatment of all prisoners accordingly."

For those prepared to look beyond the tabloid headlines, this finding was one of many warning signs over the past year of a prison service under significant strain. Basic standards of safety, fairness and decency cannot be allowed to deteriorate further. Rules matter in closed institutions. Lord Woolf made it clear in his seminal report into the disturbances at Strangeways, prisoners need reasons for rules. A civilised prison system has to be fair in the eyes of those who live and work in it.

This article first appeared on the Huffington Post.