Ensure MacBook Pro is fully asleep when travelling
Some people mention to make sure sleep light begins to pulse. I have no idea what this means. I don't know what is the sleep light and I don't see any lights pulsing.


The pulsing sleep indicator reference that you appear to be encountering in Web searches is a feature present in non-retina unibody MacBook Pro's (last released in Mid 2012) and looks like this:



Image credit: https://www.nix.ru/art/pic/web_news/2010/jul/pb1279346977.jpg

This indicator is not present in retina MacBook Pro, MacBook Air and retina MacBook.

Closing the lid indeed puts your MacBook Pro (and any other MacBook) to sleep mode. This is the default behaviour. The only cases when a MacBook Pro doesn't enter sleep mode when shutting the lid are:


It is connected to an external display. In this case, it works in so called clamshell mode.
You are using a 3rd party utility program to force MacBook to remain in awake state despite closing the lid.
When connected to a power source, the MacBook technically remains in sleep mode but can perform certain activities if Power Nap is enabled.


Even if you are not putting your MacBook Pro to sleep mode explicitly by using  → Sleep command, closing the lid puts it in sleep mode by default. It is quite common practise among MacBook users and some folks (including yours truly) seldom shut down their MacBook Pro.

There should generally be no risk in carrying it in your backpack. However, it would be advisable to watch (hear) for any fan noises and look (feel) for any heat to die down before tucking it in.