Well, there’s a lot of experts with complicated methods...but I just go by the trichome colors. I have harvested when clear, when milky, and when ~30% amber. Bottom line...I like my Sativa milky and my Indica 30% amber.
This is solid advice. I can’t find any references right at the moment but there’s many solid places on the internet saying that for a couchlock kind of high you wait until a little longer to harvest. I forget the REAL chemical reaction that takes place but it’s something like the THC breaks down to CBN which contributes to the couchlock feeling.
So Indica dominant plants are already known for their couchlock effects and the ability to put one to sleep. This effect is then enhanced when you give the plant more growing time and allow those THC molecules to degrade to CBN. Also why we call them INDA-COUCH.
Conversely, if you harvest earlier, you have less quantities of THC and CBN present in the trichomes which doesn’t give you that couchlock high, but rather a quite uplifting high.
So you can further promote the ‘SILLY SATIVA’ effect by harvesting a little earlier than normal.
You should really read up on the way the molecules break down from one thing to another and how each contributes to being sedated. Once you know that and know a little bit about the strain you’re growing you can really start playing with harvest times and see what seems to suit you best. You want to go sink into a couch and pass out; harvest later. If you want to be able to smoke and go about doing activities; harvest a little earlier.