At 36, I reduced to 7 sessions to minimise this effect. Working 8 sessions, I was paying a month's drawings a year in "penalty tax" on a pension I may or may not get in 30+yrs. Bonkers.
The pension rules are completely perverse- I've dropped a session after the accountants pointed out an effective 84% marginal tax rate once the charge for exceeding the £40k annual allowance is taken into account. Hourly rate less than our newly qualified nurse? No thanks- I value my time and sanity.