Time that macro takes to run increases each time.

I have a bunch of procedures that I use to process a spreadsheet full of data.

Each time that I run these, on the same set of data, the time that it takes for the macro to complete iit's processing increases.

Closing all open spreadsheets, then re-opening them, in order to run the macro again has no affect - the duration continues to increase from the longest time that it took previously.

Closing, then re-opening the Excel application itself though, resets the duration of these procedures back to a relatively short amount of time.
The duration will then again continue to increase each time that the macro is run.

Please can anybody point me in the right direction of the sort of things that may cause something like this??

Any ideas or advice would be greatly appreciated.

Many Thanks,

Tony