I've experimented with A LOT of ways to manage my time.
Traditionally I would think about how long a task would take me to complete and then use time blocking to schedule it into my day. But some days I would smash through some pretty big tasks and still be fresh at the end and other days I could hardly complete one relatively small task.
Because some tasks require more energy and have nothing to do with time
Every task can be categorised in one of two ways:
A 'Working It Out' Task. These are more draining and need more brain power. They require a lot of energy and momentum to get done and if you've got a lot of them in your day you're going to walk away completely drained.
A 'Getting It Done' Task. These ones you already have figured out and you can get in and get them done. You just need to ride the momentum energy wave all the way down the hill
Take writing for instance. Writing itself isn't necessarily a working-it-out task. The working it out tasks are searching for an idea, doing a brain dump, or getting the research done. If that is done, the writing is actually a getting it don task that can happen lightning fast.
How does this work practically day to day?
Stop trying to manage your time and manage your energy instead.