During the post-Guru period of the 18th century as groups of Sikh warriors under the command of a number of regional chieftains (Misls) engaged in a war for survival and eventual Sikh sovereignty in Punjab against hostile Mughal and Afghan forces, we find the very first references to Sikhs wearing helmets.
One of the three oldest chronicles of early Sikh history during the post-Guru era is the composition Sri Gur Panth Prakash written by Rattan Singh Bhangoo started in 1808 and completed in 1841. Rattan Singh was well acquainted with Sikh history being the grandson of famous Sikh warrior, Mehtab Singh, who had killed Massa Ranghar and his chronicle based on family history and Sikh oral tradition provides important insights into the Sikh struggle for survival during the Misl era.
Chronicling the Sikh battles with the enemy forces of Afghan Ahmed Shah Durrani who invaded Punjab a number of times between 1848 and 1869, Rattan Singh mentions that the Afghans who were havily armoured wore helmets:
As his troops left Lahore (on their way to Delhi),
They consisted of the most muscular giants of men,
They boasted, “where are those so-called brave Singhs,
We would like to have a glimpse of them.” (2)
Huge horses had they, being themselves very tall,
They were being led by the highest Pathan chiefs.
With steel helmets on their heads, bodies wrapped in steel armour,
They had steel coverings even around their legs. (3)
With feet in steel shoes and hands in steel gloves,
They had steel covers even on their horses.
These Gilja sect Pathan soldiers launched an expedition,
With the assistance of Pathan soldiers of Kasur. (4) 
An early reference to a Sikh wearing a helmet appears as Rattan Singh chronicles how a Sikh warrior named Sukha Singh donned armour, including a helmet as he prepared for battle with a Pathan:
Remarked Sukha Singh, “Blessed are you, my dear boy,
Now see how I deal with that audacious Pathan?”
With these words Sukha Singh sent for his steel armour,
And covered his head and hands with steel helmet and gloves. (30)
He saddled his horse covering it with a steel armour,
Which he had once snatched from a Pathan soldier.
Charat Singh helped him in wearing that armour,
As well as decorated Sukha Singh’s horse for an encounter. (31) 
Rattan Singh also chronicles another armoured, helmet wearing Sikh warrior named Karam Singh who aided Nawab Dina Beg of Jalandhar in his fight with the invading Gilja Pathans of Ahmed Shah Durrani:
There emerged a Singh warrior from the S. Sham Singh’s contingent,
S. Karam Singh was his name from the village of Paijgarh.
There did he arm himself with armour and battle dress,
After he had finished with his ablutions and daily routine. (48)
Thereafter, did Karam Singh mount his horse,
As he resolved to fight for his faith and ideology.
He was determined to settle scores with the desecrators,
As he never wished to return without accomplishing this task. (49)
After wearing his coat of arms did he wear a steel helmet,
As after slinging a sword did he wield a shield in his hand.
After this, spurring their horse, did he make a move,
As he took along those who were dear and loyal to him. (50)
The spot where there was the highest concentration of Gilja Pathans,
There did these Singhs arrive to launch an attack.
As the Pathans were protected by a striking force in the front,
The Singhs attacked those who were in the vanguard. (51)
As this contingent in the vanguard spotted the advancing Singhs,
They attacked the Singhs with a lightening speed.
As the sword of one of them touched Karam Singh’s head,
His turban along with the steel helmet came down. (52)
Karam Singh, being bare-headed without his turban,
Did ply his sword on his Mughal rival’s head,
With a bound did the Pathan fall flat on the ground,
As Karam Singh’s sword had cut through his neck. (53) 
Although this evidence suggests Sikh warriors of the second half of the 18th century did wear helmets, their use seems to be very rare as was access to armour in general. In chronicling the story of Gurbakhsh Singh and his band of 30 Sikh warriors who were martyred at Darbar Sahib at Amritsar in December 1764 fighting against an invading army of Gilja Pathans of Ahmed Shah Durrani, Rattan Singh describes the Sikhs as wearing turbans and also makes an important observation regarding their lack of armour:
On the other side had Gilja Pathans made elaborate preparations,
As they had heard about concentration of Singhs in large numbers.
Gilja Pathans had covered themselves with steel helmets and armours,
But Singhs had hardly any armours to protect themselves. (49) 
Existing visual evidence concurrs with this observation that the use of armour among the Sikhs seems to have been limited during the 18th century and early parts of the 19th century, or if it was worn, it would mainly consist of body armour and not helmets. In two rare early 19th century paintings of Sikh battle scenes, we can see that the Sikh soldiers all wear traditional turbans rather than helmets and no body armour, other than the traditional Sikh shield carried on the back. When armour was worn, it traditionally consisted of the Char-Aina body plates, such as those depicted in the rare 1780 painting of a son of Sikh chieftan Jassa Singh Ramgharia or the chain mail body armour worn by the turbaned Sikh soldiers depicted in the 1770-1780 illustrated folio of a battle scene from the manuscript Tawarikh-I Jahandar Shah.
1. Rattan Singh Bhangoo, Kulwant Singh Translator, Sri Gur Panth Prakash, Volume II, (Chandigarh: Institute of Sikh Studies, 2010), 181
2. Ibid. 187
3. Ibid. 427
4. Ibid. 641