I would completely remove eq. (1), as in my opinion it makes no sense here. In eq. (3) looks like only 1 is to power t/7 - some formatting needed, maybe (1/2) instead of 1/2 will help.
7 is 7 days, arbitrary chosen half life.
I have 15 nightshifts left in a row, so sadly little time to engage :(
hey hotbit, 15 nightshifts in a row! may i ask what is your job? i implemented the brackets and th instead of 7, however i left eq. (1) just to give the info that in literature this is often seen.