Lecturers are being left vulnerable and anxious for their futures due to use of casual contracts by universities, campaigners claim.
The University and College Union estimate half the UK's higher education staff are not on secure long-term contracts.
One academic, paid hourly at a Welsh university, said the uncertainty was "stressful".
A body representing universities said there were practical reasons for them.
The UCU said many teachers and lecturers were being placed on short-term and open-ended contracts, without a guaranteed set number of hours or set level of pay.
It said the lack of stability was part of a general decline in working conditions for staff at colleges and universities, and has launched a UK-wide campaign calling for an end to "casualisation" of higher education contracts.
'You don't know when you're working next'
One academic at Cardiff University, who did not wish to be named, said she felt stressed and anxious on her contract which does not guarantee a set amount of hours.
"It's quite stressful because you have an open-ended contract but without guaranteed hours or a fixed income," she told BBC Wales.
She said that while she could earn a good salary one month, by working a large amount of hours, the next month her wage could be a lot less, and then in the holidays she would earn nothing at all.
"You're feeling in a really temporary and fragile situation because you're never knowing if you're going to work or not from one term to another or from one year to the other," she said.
Renata Medeiros, a UCU official at Cardiff University, was on a casual contract before securing a permanent position.
She said it was hard for staff to plan lectures and courses when they did not have a guarantee of work next term.
"Their ability to think long term, to develop their careers is obviously limited… people feel vulnerable in those positions", she said.
"It's obviously not good for the students either - it's this sense of belonging and being part of the institution that these people on precarious contracts don't really gain.
"Their ability to deliver the excellent teaching that the university prides itself on is very limited".
'Necessary in certain circumstances'
The Universities and Colleges Employers' Association said the overwhelming majority of teaching and research staff were employed long-term.
But it said some fixed term or variable contracts were necessary in certain circumstances, including when universities employ academics from outside the sector or when giving postgraduate students teaching opportunities.
A Cardiff University spokesperson said the university was a living wage employer and does not have zero-hour contracts.
But the spokesperson said it contracted a range of people in different ways, adding that some work was predictable and long-term while other work related to specific projects which have a time frame or short term.
"The vast majority of our staff, both academic and professional services, are employed on open-ended contracts and enjoy generous terms and conditions when compared with other sectors," the spokesperson added.