Why are my seedlings tall, thin and falling over?

They are stretching for light. Seedlings that come up fast and then go tall, pale and thin are not overfed or overwatered. They are reaching, and a windowsill in spring is much darker than it looks to us.

The likely cause

A seedling that cannot find enough light invests everything into height, gambling that a few more centimetres will lift it above whatever is shading it. Stems go long and weak, leaves stay small, and eventually the whole thing topples. Warmth without matching light makes it worse, because heat speeds up growth that the light cannot support.

How to fix it

1. Give them far more light. A simple grow light a few inches above the tops, running most of the day, beats any windowsill. If you are staying with the window, use the brightest one you have and rotate the tray daily.
2. Bring the temperature down. Once seeds have germinated, cooler growing produces stockier plants. Take them off the heat mat.
3. Give them a breeze. A fan on low for a few hours a day thickens stems. Brushing them gently with your hand a few times a day does much the same job for free.

What to expect

Tomatoes are forgiving here: bury the long stem deep when you pot them on and they will root along its length and come back stocky. Basil will not do that, so fix the light now and expect firmer growth within two weeks.