Relative humidity higher when lights off? Help me lower it please!

Hi

I am about 5 weeks into flower and took a look at my hygrometer when the lights first came on last night and realized the relative humidity is at 70%!!! After the lights are on it gets down to about 40%. I am running a 600w in an aircooled hood and only exhausting when the light is on. Do I need to buy a dehumidifier or if I run my exhaust during lights off with this lower the relative humidity?

Thanks

because your only exhausting when lights are on should have it 24/7 good airflow is key imo otherwise mould will occur

yep agree with Redeye! Keep your fans and exhaust going 24/7