yellowing spinach

Hello there, it's been a while since my last post on here.

I have some spinach that has started turning yellow. We've already had quite a bit of good green leafage off of it, and I was wondering whether it was now the natural turn of events for spinach to start yellowing before dieing. I'm concerned because it is in a spot where there is not a great deal of sun, but there is some. It's hot here, in Japan, so definitely not the heat.

Anyone else had yellowing spinach? What do you think the cause was?

Thanks