SAD and/ or Meltdown.

I think maybe you are getting the terminology mixed up. 

A meltdown is a short lived thing, an explosion of emotions which normally has at least one trigger. I would say a few hours at the most, although the exhaustion afterwards can last a few days. There is an explanation here https://www.autism.org.uk/advice-and-guidance/topics/behaviour/meltdowns/all-audiences

Thank you - I hope posters read this and change Meltdown to Burnout.

Burnout would make more sense. However if it is something that happens every year I think that suggests that the SAD diagnosis is likely correct. Have you tried one of those artificial daylight lamps or taking vitamin D supplements in the winter?

I just posted this link on another discussion https://theautisticadvocate.com/2018/05/an-autistic-burnout/ It's one of the best articles I've found on autistic burnout. Towards the end of the article it gives a list of the key symptoms, which I'll copy below:

• A growing lethargy
• An increase in irritability
• An increase in anxiety
• An increase in over-sensitivity to sensory information
• A dramatic decrease in sensitivity to sensory information
• Heightened Auditory processing disorder
• A decrease in verbal language
• A decrease in text language
• An increase in Shutdowns and heightened withdrawn state
• An increase in the frequency and severity of Meltdowns
• A diminished ability for the person to self-regulate their emotional state
• The slowing down of the thought processes
• Brain fog
• Memory loss 
• A decrease in your ability to effectively communicate what you want 
• A decrease in motivation
• An inability to generate momentum of body and of action
• An increase of rigidity, narrowing of thinking
• A feeling like your vision is tighter or narrower
• Extreme forgetfulness
• Extreme overwhelm
• A massive increase in guilt
• An increase in Executive Dysfunction
• An increase in Demand Avoidance

Thank you... I have a lightbox which seems to help me as does Fluoxetine (although that perhaps treats me over the condition if you know what I mean). I attempt to get out on an early afternoon but it is so much like running through treacle.

The list is so similar to SAD which, from the NHS, are:

• a persistent low mood
• a loss of pleasure or interest in normal everyday activities
• irritability
• feelings of despair, guilt and worthlessness
• feeling lethargic (lacking in energy) and sleepy during the day
• sleeping for longer than normal and finding it hard to get up in the morning
• craving carbohydrates and gaining weight
• difficulty concentrating

From your list and hopefully more for me rather than attempting a diagnosis ...

• A growing lethargy (Demonstrated over years from fitbit info).
• An increase in irritability (Yes - as per SAD)
• An increase in anxiety (Yes - Linked to tiredness and guilt. Overly anxious anyway)
• An increase in over-sensitivity to sensory information (Lights are worse but in Winter they exist more).
• A dramatic decrease in sensitivity to sensory information (dont know)
• Heightened Auditory processing disorder (Dont know)
• A decrease in verbal language (Slur but linked to tiredness).
• A decrease in text language (Terrible at texting anyhow and a generation where one message evry 3d is acceptable haha)
• An increase in Shutdowns and heightened withdrawn state (Avoid house parties more)
• An increase in the frequency and severity of Meltdowns (Dont believe so)
• A diminished ability for the person to self-regulate their emotional state (Ties into SAD symptoms)
• The slowing down of the thought processes (Yes but linked to tiredness)
• Brain fog (as above)
• Memory loss (as above)
• A decrease in your ability to effectively communicate what you want
• A decrease in motivation
• An inability to generate momentum of body and of action
• An increase of rigidity, narrowing of thinking
• A feeling like your vision is tighter or narrower
• Extreme forgetfulness (not exactly)
• Extreme overwhelm (Not particularly)
• A massive increase in guilt (yes as per SAD)
• An increase in Executive Dysfunction
• An increase in Demand Avoidance

Thank you on the article... I will read when have time to digest 

My plan is to try and 'look after' myself as opposed to normal Summers and see if the Winter pattern remains.

Thank you... I have a lightbox which seems to help me as does Fluoxetine (although that perhaps treats me over the condition if you know what I mean). I attempt to get out on an early afternoon but it is so much like running through treacle.

The list is so similar to SAD which, from the NHS, are:

• a persistent low mood
• a loss of pleasure or interest in normal everyday activities
• irritability
• feelings of despair, guilt and worthlessness
• feeling lethargic (lacking in energy) and sleepy during the day
• sleeping for longer than normal and finding it hard to get up in the morning
• craving carbohydrates and gaining weight
• difficulty concentrating

From your list and hopefully more for me rather than attempting a diagnosis ...

• A growing lethargy (Demonstrated over years from fitbit info).
• An increase in irritability (Yes - as per SAD)
• An increase in anxiety (Yes - Linked to tiredness and guilt. Overly anxious anyway)
• An increase in over-sensitivity to sensory information (Lights are worse but in Winter they exist more).
• A dramatic decrease in sensitivity to sensory information (dont know)
• Heightened Auditory processing disorder (Dont know)
• A decrease in verbal language (Slur but linked to tiredness).
• A decrease in text language (Terrible at texting anyhow and a generation where one message evry 3d is acceptable haha)
• An increase in Shutdowns and heightened withdrawn state (Avoid house parties more)
• An increase in the frequency and severity of Meltdowns (Dont believe so)
• A diminished ability for the person to self-regulate their emotional state (Ties into SAD symptoms)
• The slowing down of the thought processes (Yes but linked to tiredness)
• Brain fog (as above)
• Memory loss (as above)
• A decrease in your ability to effectively communicate what you want
• A decrease in motivation
• An inability to generate momentum of body and of action
• An increase of rigidity, narrowing of thinking
• A feeling like your vision is tighter or narrower
• Extreme forgetfulness (not exactly)
• Extreme overwhelm (Not particularly)
• A massive increase in guilt (yes as per SAD)
• An increase in Executive Dysfunction
• An increase in Demand Avoidance

Thank you on the article... I will read when have time to digest 

My plan is to try and 'look after' myself as opposed to normal Summers and see if the Winter pattern remains.