You know what it's like when a child, toddler or baby is hungry! They let you know it, loud and clear - they become irritable, their concentration span lessens and they can be difficult to soothe. I often liken horses to children (you wouldn't yell at your child or strike them or punish them when they didn't understand something!) when talking about patience, kindness and understanding.
So, like a child, a hungry horse can be a challenge to teach and they can show anxious, disrespectful and fidgety behaviour. During clinics I encourage people to bring plenty of hay for their horses as I know that around the 90 minute mark they start to look for food and lose concentration (this is all the reason to keep your learning sessions short and take frequent snack breaks).
I say there are no 'naughty' horses ... they are only doing the thing that makes most sense to them at any given time!
Driving around to lessons this past week, I've seen many horses standing in bleached paddocks with hardly a stem of decent forage. What is there, is sour and lacking in nutrition and the horses have a vague, hopeful look about them when they see me stop to have a look. Horses need about 2% of their bodyweight in forage (grass,hay) a day, so weigh your horse (use a weight tape) and calculate what he/she needs.
Crankiness can be plain old hunger ... so make sure your horses' daily food needs are being met before you blame him/her for being cranky!